Analogue audio frequency counter - Page 1
Forum section: electronics / projects

author: sv3ora
author_rank: Regular Contributor
timestamp: February 20, 2021, 09:55:34 pm
content: Hi I have found this frequency counter circuit which worked ok but I have a problem.The range I am interested is 480Hz-2.7KHz only. Can I transform the circuit somehow to cover only this range, so that the meter spreads throughout it's whole scale instead of part of it, so as to increase the resolution of the meter?Also I have no idea how the meter works, perhaps if I knew I could derive the modifications myself.Any other such analogue meters with discrete components? I cannot find any on the net. Maybe you remember one from old magazines?

author: srb1954
author_rank: Super Contributor
timestamp: February 21, 2021, 02:13:42 am
content: Quote from: sv3ora on February 20, 2021, 09:55:34 pmAdjusting the high frequency end is easy - just install a slightly lower value (approx 75nF) in place of C4.Adjusting the lower end is more difficult. You could possibly put a silicon diode or 2 in series with the meter. That would cut the lower end at the expense of some non-linearity of the meter readings. The performance of the whole circuit is highly dependent on the meter characteristics so definite design decisions can't be made without knowing the full meter details.This is a really ancient circuit. You could do a much simpler design using a small microprocessor to count the incoming frequency and output a suitable PWM signal to the analog meter. With S/W calculations you could scale and offset the readings any way you liked.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2021, 04:57:54 pm
content: The modern version of that circuit is a charge pump frequency-to-voltage converter or frequency-to-current converter in this case.The zero can be offset by directing a fixed current into the collector of TR3 from the positive supply. For reasons of voltage compliance, this also means adding a diode in series with the meter circuit to raise its forward voltage drop which will *not* affect calibration because the circuit is dealing with charge and current instead of voltage.

author: wizard69
author_rank: Super Contributor
timestamp: February 22, 2021, 05:12:29 pm
content: Is there a reason for this odd range analog meter to be analog? My first thought is that this would be easy with a microprocessor. if you go microprocessor why not go digital display.Doing so would make calibration and range adjustment very easy. I'm not sure how accurate you need to be but you specifically mentioned increasing resolution. That will do you no good if you can't calibrate the meter afterward. Maybe I'm missing something here because at the other end of this is the fact that you can buy counter modules dirt cheap these days.

author: sv3ora
author_rank: Regular Contributor
timestamp: February 22, 2021, 10:40:31 pm
content: Quote from: David Hess on February 22, 2021, 04:57:54 pmI am not sure I understand how to do it properly. a simple resistor from the vcc to the collector of the meter driver transistor? And also a diode in series with the meter with the anode connected to the meter? Is that right?

author: TimFox
author_rank: Super Contributor
timestamp: February 22, 2021, 10:55:18 pm
content: One of the gems of my collection is a General Radio 1142-A "Frequency meter and discriminator", from 1961, usable up to 1.5 MHz. See manual:https://www.ietlabs.com/pdf/Manuals/GR/1142-A%20Freq%20Mtr%20&%20Discr.pdfIt uses a charge pump to produce an analog current proportional to the frequency, which is displayed on a large d'Arsonval meter that reads from 0.0 to 1.5 . To improve the resolution, a 16-position rotary switch subtracts a precise current from the analog current and and the meter displays the difference, thus increasing the resolution by a factor of 10 (while limiting the display range).The unit is very accurate when using the current output (0.05% of full-scale plus 0.05% of reading), but the meter reading accuracy is degraded by using a 1N914 diode (which is temperature sensitive) in a passive circuit to give a non-linear meter scale (to 1% of reading, when above 10% of full-scale). The analog output can be used as a .discriminator to follow frequency modulation of the input signal.You are not likely to duplicate the tube circuitry, but you might use the circuitry in the "interpolation" offset to work with your charge pump. The unit is calibrated against twice the line frequency (120 Hz on 150 Hz full-scale), and relies on precision capacitors (GR was good at that) for the other frequency ranges.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2021, 07:27:34 pm
content: Quote from: sv3ora on February 22, 2021, 10:40:31 pmA simple resistor will not work because the voltage at the collector changes with signal level to drive the meter. For reasonable precision, it will require a current source or some other circuit change like adding a cascode transistor to make the voltage at the collector constant so that a resistor can be used. The diode would only be required if the current source solution requires a voltage higher than the voltage across the meter to operate, which is likely.

author: sv3ora
author_rank: Regular Contributor
timestamp: February 23, 2021, 08:26:59 pm
content: Another idea I have is to drive a negative pulse triggered monostable with the negative pulses of the pulse counter.Then set the RC in this monostable to produce the maximum pulse at the highest frequency.I wonder how well this would work in comparison to the standalone pulse counter.