Due to the dynamically updated fleet of cars (foreign cars) in our country, getting a digital scale unit (DS) of an old car radio or tuner for a radio amateur does not present any particular difficulties.
Most often, these digital switches were implemented on a Sanyo LC7265 microcircuit paired with an LB3500 divider in a single digital block connected (rigidly or flexible cable) to an indicator unit, and were intended to indicate the received frequency in the AM MW-LW (AM on NE-LW) ranges. and FM (FM VHF). According to intermediate frequency standards, the LC7265 has “hardwired” the possible options for their selection (see Tables 1, 2) by reconnecting pins 11 – 15 with an indication step of 1 (10) kHz in the AM range (0 - 1990 kHz) or 50 kHz in the range FM (0 - 199.5 MHz).
In their designs, radio amateurs use these units either for their intended purpose - as a digital scale, most often for an FM receiver, and in the bands not only FM1, 2, but also others, starting with the civil CB band of 27 MHz, with a step of 50 kHz.
Less commonly, this central frequency is used as a frequency meter. The readings are read from the indicator block and the selected IF value is added to them (and in the FM range it can be subtracted), which is not entirely convenient. And the indication step of 50 kHz, if the FM frequency range is selected, does not allow the frequency to be measured accurately enough. On the AM band with an acceptable step of 1 kHz, the upper limit is limited to 2 MHz.
Actually, this means that when starting a measurement, you need to know in what range (how many MHz) the frequency being measured is located. That is, it turns out that after the first section the range up to 18 MHz is divided into sections of 2 MHz (from 0 to 1999 kHz). In this case, the frequencies of sections above 2 MHz with even values (megahertz) will always be indicated by the first digit of the indicator - one.
Thus, the frequency measurement algorithm can be represented in two stages:
1. First, on the FM range, we determine the frequency of the signal under study with an accuracy of +/- 50 kHz. For example, the indicator will show 14.00 MHz. The actual frequency will be 14.00 - 10.7 MHz (programmed IF) = 3.3 MHz.
2. Next, we carry out measurements in the AM range. The indicator will show only the last three digits of the measured frequency value in kHz + 455 kHz. Let's say 378 (kHz). Conclusion: the measured frequency is 3.378 MHz + 455 = 3.833 MHz.
If on the FM band the first of the four digits is even, then during clarifying measurements on the AM band the first digit of the indicator (one) should be ignored. For example, 15.00 (indicator shows) – 10.7 (subtract IF) = 4.3 MHz (the first digit “4” is even). At the second stage of measurements, the indicator will show 1378. The measured frequency will be 4.378 MHz (the unit is ignored, i.e. replaced by 4) + 455 kHz.
The frequency of 455 kHz (or another, there are standard options, see Table 2) is “hardwired” into the digital signal from the car receiver. This is calculated on the fact that in the receiver itself the IF = 455 kHz (or another...), and when working in conjunction with the receiver, the display will show the true indicators of the frequency received by the receiver.
The algorithm is as follows: in the receiver F ink = Fsign. - Fgpd (always the same IF = 455 kHz, because the VFG is also tuned, the F signal changes. Next, Ffd is detected in the audio spectrum and ultrasonic frequency).
In the digital signal switch it’s the same, only the frequency of 455 kHz (“analogue of the receiver’s Fgpd”) is hardwired into the microprocessor of the central switch and does not change. In this case, when changing (tuning the receiver) in frequency Fsign. the display will show the changing reception frequency according to the Fdispl algorithm. = fsign. - Fprotected.
If you take the DS separately (outside the receiver) and apply any frequency to its input (frequency meter mode), then in order to obtain (correctly read) the value of the measured frequency, you need to add (sum up) 455 in your head to the display readings. After all, in the CS these 455 kHz are “hardwired” and they are taken into account in the readings on the display.
A way out of the situation (in order not to count) can be the use of a reference generator (OG) with a simple mixer. In the exhaust gas, you can use a piezoceramic resonator at 455 kHz (it can be found in many imported point-and-shoot cameras). Without a signal at the mixer input, the TsN indicator will show 000 kHz. When the measured signal is applied to the mixer input, the frequency will be indicated in 1 kHz steps up to the upper limit of 1999 kHz. Then 000 kHz will follow again, and so on up to 18 MHz. This happens because the counting and indication of high-order digits (megahertz in the AM range) in a digital scale above one is not carried out.
Thus, in order to “level out” these 455 kHz “wired” into the central frequency frequency, you can make an attachment in which the frequency of 455 kHz is summed in the mixer (it is obtained in the exhaust exhaust of the attachment using a 455 kHz resonator) with the frequency of the measured signal. Then the display will show numbers corresponding to the measured frequency, and there is no need to add up in your head. Of course, taking into account the error of the resonator in the exhaust gas of the set-top box, the “climbing” of its signal to the input of the central switch, the amplitude and type of the input signal and the exhaust signal, the drop in frequencies at HF, and much more when designing the device.
Below is a diagram of the CS (Fig. 1), only slightly different from that shown in.
Fig.1
As follows from Tables 1 and 2, switching the pins of the LC7265 microcircuit allows this digital frequency switch to operate with intermediate frequencies of +455 kHz and -10.7 MHz.
Using the measurement technique described in the article, you can, of course, do without a mixer with exhaust gas by performing two simple arithmetic operations... Often this is enough, and the accuracy is quite satisfactory for the radio amateur (CN step = 1 kHz).
Moreover, when carrying out frequency measurements, when the accuracy of scale readings with a step of 50 kHz is sufficient (for example, in the VHF range with FM), you can limit yourself to only the first point of the algorithm, again, without using a relatively low-frequency mixing attachment. In this case, the upper measurement limit can theoretically reach 199.5 MHz.
Of course, to measure frequency with a homemade (converted) device (less accurate, but more convenient), you can use the conversion method described in the article"A simple frequency meter from a Chinese receiver"
We propose, using the principles discussed in this article and using the circuitry of such converters, to make an attachment. To begin with, we advise you to pay attention to these works:
RF attachment for oscilloscope
Device for tuning quartz filters
Sources:
1. A. Romanchuk. TsSh for the receiver. – Radiomir, 2002, No. 6, p. 8.
2. S. Efimenko et al. A set of microcircuits for indicating the tuning frequency of a radio receiver. – Radiomir, 2001, No. 8, p. 40.
3. http://www.datasheetpdf.com/datasheets/Sanyo/lc7265.pdf.html
PS.The article has been re-edited taking into account the wishes of site visitors and with the consent of the author of the article27 . 01 . 2011 G.
Simple pocket miniature VHF-FM receivers with a digital scale “Manvo”, “Palito”, “ESB” and similar ones are of certain interest, since the built-in electronic scale is nothing more than a frequency meter with a digital display. By making a simple modification, you can get a frequency meter that displays hundreds, tens, units of megahertz and hundreds of kilohertz on a four-decade indicator.
Simple pocket miniature VHF-FM receivers with a digital scale “Manvo”, “Palito”, “ESB” and similar ones are of certain interest, since the built-in electronic scale is nothing more than a frequency meter with a digital display. By making a simple modification, you can get a frequency meter that displays hundreds, tens, units of megahertz and hundreds of kilohertz on a four-decade indicator.
Small dimensions, high efficiency (current consumption is only a few milliamps) and a wide range of operating frequencies (up to 800 MHz!) make such a measuring device quite attractive.
Radio receiver circuit.
It consists of (Fig. 1):
.Radio receiving device (RPU) board on the SC1088 (or TDA7088) microcircuit, transistor ultrasonic frequency unit and two transistor ultrasonic frequency unit.
.The second board contains a clock, elements of a digital scale (frequency meter) and control buttons.
The supply voltage is constantly supplied to the clock unit and when the receiver is turned off, the current time is displayed on the display. When the receiver is turned on by switch SA1, the supply voltage is supplied to the receiver and the frequency meter control bus. The local oscillator signal is amplified by the RF frequency meter, sent to the frequency meter, and the tuning frequency is displayed on the indicator.
The receiver is built using a superheterodyne circuit (lower setting) with a low IF (70 kHz), and therefore, to correctly indicate the frequency settings, the frequency meter readings are overestimated by 0.1 MHz, which must be taken into account when making measurements. Obviously, if you apply a controlled signal to the input of the frequency meter, then when certain conditions are met, its frequency will be indicated.
First of all, to do this, you should install a small-sized high-frequency socket (for example, SMA) on the receiver body, placing it closer to the frequency meter input. In addition, to turn on the frequency meter, you need to install a small switch (in the diagram it is designated as SA2").
The PD9-2 switch is installed (glued to the board) next to the volume control; for this, jumpers J11, J14 and capacitor C11 (the numbering is given in accordance with the designation on the board) must be installed on the side of the printed conductors. The switch body is connected to a common wire. The SMA socket is installed on the narrow side next to the ribbon harness J21, which goes from the receiver board to the clock (frequency meter) board. The central contact of the socket is connected through a capacitor with a capacity of 500... 1000 pF to the input of the frequency meter or RF amplifier, and the housing is connected to the common wire.
The RF circuit diagram is shown in Fig. 3.
Since it has two stages, three connection options are possible:
.to the input of the first stage (point 1),
.to the input of the second (point 2)
.or to the frequency meter input (point 3).
It is clear that the connection location will affect the operating frequency range and sensitivity of the frequency meter, but in any case, a signal voltage of more than 1V should not be applied. For example, when connecting the measured signal to the input of the first stage, the sensitivity in the frequency range up to 100 MHz is less than 1 mV. It should be noted that with this connection the sensitivity is excessive and leads to the fact that the frequency meter will be too sensitive to interference and interference. In addition, in this range, due to nonlinear effects in the amplifier, distortion may occur and the frequency meter can indicate the frequency of the harmonic components of the signal. If the frequency meter does not respond to interference, then if there is no signal, the indicator will display a reading of 000.1 MHz.
In the author’s version, point 3 was chosen for connection. In this case, an additional switch is connected between the battery positive (jumper J23) and the frequency meter control bus (see Fig. 1).
To do this, the red (or third from the top) wire in harness J21 must be disconnected from the receiver board and connected to the switch. This connection allows you to turn on the frequency meter when the receiver is turned off or turn it off when the receiver is on. The latter is also convenient because when receiving a radio station, the frequency meter can be turned off and the current time can be monitored.
The lower limit of the measured frequency is 0.5... 1 MHz, the upper limit depends on the supply voltage and for 2.5V it is 600 MHz, for 3V it is 700 MHz, and at 4V it reaches 800 MHz. More voltage should not be applied.
When the receiver is turned off, the current consumed by the frequency meter (along with the clock) depends on the measured frequency and varies from 0.3 mA in the absence of a signal to 0.7 mA at frequencies up to 50 MHz and up to 4 mA at 600 MHz.
Source: Radio magazine No. 2, 2003.
From the receiver "PALITO" PA-618.
Models of such receivers contain a built-in digital frequency meter, which, thanks to the presence of a system for automatically tuning and maintaining the local oscillator frequency, significantly improves the performance of the receiver. In addition, the low intermediate frequency of the receiver (70 kHz) greatly simplifies its pairing with a frequency meter, since it is possible to connect the latter directly to the local oscillator using only buffer amplifiers.
Usually they are two transistors connected in an OE circuit.
These amplifiers provide sufficient frequency counter sensitivity to be used as a standalone device. It allows you to measure frequencies from 1 to 150 MHz with an accuracy of tenths of Hz, and with a sufficiently high signal level - up to 300 MHz.
True, its accuracy is relatively low, but receivers are so cheap that you can put up with both low accuracy and a not very wide range of frequencies measured by such a frequency meter.
In addition, it is worth considering that in amateur radio practice it often happens that this particular range is needed.
The easiest way to use a receiver's digital scale as an independent frequency meter is to disconnect it from the local oscillator and connect it to the signal being measured.
But at sufficiently high frequencies (from about 20 MHz) and a sufficiently large signal, another method can be used. It is enough to disconnect the capacitor from the local oscillator circuit and bring the circuit of the device whose frequency needs to be measured closer to the local oscillator coil.
By the way, if you install a toggle switch on the receiver body that turns the capacitor on/off, and solder a needle-shaped probe to it, as shown in Fig. 1, then subsequently the receiver can be used, without disassembling, both for its intended purpose and as a frequency meter.
In the body of the marker.
Only four cable wires need to be unsoldered from the receiver and soldered to the assembled RF amplifier.
(parts for which can be taken from the receiver). R6 - so that the readings do not flicker.
Datasheet: SC3610
The input capacitance (10pf) can be reduced to 1pf in order to reduce the introduced error in the case of direct connection to an oscillatory circuit.
The frequency meter can also be used as a clock, you just need to supply power through the switch and use the free leads to correct the time, see photo
Source of information: forum topic - “Converting a Chinese radio into a frequency meter”
If we are to take on the creation of a digital frequency meter, then immediately make a universal measuring device capable of measuring frequencies not up to a couple of tens of megahertz (which is typical), but up to 1000 MHz. With all this, the scheme is no more complicated than the standard one, using pic16f84. The only difference is in the installation of the input divider, on a specialized chip SAB6456. This electronic meter will be useful for measuring the frequency of various wireless equipment, especially transmitters, receivers and signal generators in the VHF bands.Frequency meter specifications
- Supply voltage: 8-20 V - Current consumption: 80 mA max. 120 mA
- Input sensitivity: max. 10 mV in 70-1000 MHz range
- Measurement period: 0.08 sec.
- Information update rate: 49 Hz
- Range: 0.0 to 999.9 MHz, resolution 0.1 MHz.
Features and advantages of the scheme. Fast operation - short measurement period. High sensitivity of the input signal in the microwave ranges. Switchable intermediate frequency offset for use in conjunction with the receiver - as a digital scale.
Schematic diagram of a homemade frequency meter on PIC
Frequency Meter Parts List
R1 - 39k R2 - 1k
R3-R6 - 2.2 k
R7-R14 - 220
C1-C5, C6 - 100-n mini
C2, C3, C4 - 1n
C7 - 100 units.
C8, C9 - 22 p.m.
IC1 - 7805
IC2 - SAB6456 (U813BS)
IC3 - PIC16F84A
T1-BC546B
T2-T5 - BC556B
D1, D2 - BAT41 (BAR19)
D3 - HD-M514RD (red)
X1 - 4.000 MHz quartz
All the necessary information on the microcontroller firmware, as well as a complete description of the SAB6456 chip, are in the archive. This scheme has been tested many times and is recommended for independent repetition.
The most suitable model for conversion is Palito PA-218. The receiver contains a specialized SC3610D microcircuit, which contains a frequency meter + LCD controller + clock with alarm clock. Converting the receiver into a frequency meter will take about half an hour (including coffee and a smoke break). In fact, you just need to remove the unnecessary elements - the receiver chip IC2, two resistors R5 and R13, capacitor C25 and transistor Q7. Connect the wire with the “cracodile” to the common one, and solder the wire to the capacitor C19 onto a probe-needle attached to the edge of the case (you can simply fuse a medical metal needle). Of course, if you wish, you can leave the receiver, but it will be necessary to eliminate the influence of the local oscillator on the input of the frequency meter in the measurement mode. I won’t say much about other models, but the Palito 214 was also redesigned with a different chip and worked no worse.
So what can the resulting device be used for?
1. Determine the generation frequency of any quartz from 500 kHz to 200 MHz (if such exist). I had a circuit with 49 MHz quartz at hand - the device stably determined the frequency without disrupting the generation.
2. Measure the IF and output frequencies of 40 MHz radiotelephones (to measure the output frequency, the common wire does not need to be connected).
3. The frequency range is up to 200 MHz (depending on the production date, individual copies can measure up to 400 MHz). Therefore, it is possible to evaluate the performance of HF paths of 200-300 MHz radiotelephones.
Of course, the measurement error (0.1...0.2 MHz) does not allow making precise adjustments. The device is more intended for assessing the performance of a component or device as a whole in the absence of an oscilloscope at hand or at high operating frequencies.
If you have any questions, write to [email protected] Vyacheslav.
I wish everyone good luck.
In ancient times, I purchased this SV-HF-VHF radio receiver :
The advantage of such a receiver is its digital frequency scale. As it turns out, such a device can easily be turned into a very accurate frequency meter for the range tens to hundreds of megahertz!
By unscrewing a few screws and unlatching the latches, you can open the receiver housing. Then unscrew more screws and remove the board. So, in front of them there are three parts - a back cover with batteries (1), a radio receiver board (2) and a front cover with an indication board and a frequency meter(!) (3): 
From the display board to the receiver board itself there is a group of three wires, which are labeled " A.M.", "FM" And " FM.G".
We are interested in the wire with the signature " FM" - it is soldered to the disk capacitor on the receiver board. This wire is the input wire of the frequency meter - carefully(!) unsolder it from the capacitor, because the radio receiver will still come in handy: 
Now turn on the " FM"(VHF), by moving the slider, you can use a capacitor with a capacity of several picofarads to connect it to the signal source whose frequency you want to measure. You can also check the frequency of the radio transmitter signal by placing its antenna next to the wire from the frequency meter.
But there is one caveat - the frequency meter is designed to measure the local oscillator frequency, which in this receiver 10.7 MHz higher signal frequency (intermediate frequency ( IF) is 10.7 MHz). Therefore, to determine the true frequency of the signal, you need to add 10.7 MHz to the displayed frequency.
I checked the functionality of the improvised frequency meter by bringing a transmitter with a signal frequency of 433.92 MHz to it: 
Voi la:-) As you can see, the frequency is displayed 423,3 MHz. Add 10.7 and get 423,3 + 10,7 = 434 MHz (the difference from 433.92 is 0,02 % !!!). The experiment of converting the receiver into a frequency meter was successful!
The counter turned out to be a ring one, i.e., for example, the receiver readings 998,0 MHz correspond to frequency (998,0-1000) +10,7 = 8,7 MHz.
