Am receiver frequency readout

[Rascal]

I have a 1960's un-cased 'home brew' valve radio receiver that covers 60mhz to 186mhz. This is tuned by a mechanical mult-turn pot.

I have decided to build the receiver into a case. Unfortunately it doesn't have a frequency readout so I thought it would be an idea include some kind of indicator to do this. I require something that gives me an idea of the frequency it is tuned to - to the nearest 5mhz would be sufficient.

Preferably I would like to use an analogue meter to achieve this. I have a connection from the local oscillator (frequency + IF of 21.4mhz) - would it be possible to use this?

Any ideas or suggestions would be appreciated.

Regards Paul

[Refugee]

Would a prescaler followed by a 1960s car rev counter circuit do the job?

[Rascal]

Would a prescaler followed by a 1960s car rev counter circuit do the job?

Interesting idea - I will investigate this.

I will need to discover how much I need to divide the LO by to feed the tacho circuit

Thank you

Regards

Paul

[Refugee]

The standard tacho circuit takes in a variable frequency pulse train and converts it into constant length pulses so that it is a linear analogue output when driven into a meter. The prescaler just brings it down to a frequency that is easy to work with.
The pulses just have to be short enough that they do not overlap at the high end yet long enough that they do not suffer inductive losses.

[pwdrive]

Prescaler fed to integrator then DC to MC meter, or for your 5Mhz tuning resolution why not a Xtal marker, a 5 Mhz Xtal harmonics should get up there ok.

Lawrence.

[Nick]

You need to divide by 2^17 or 2^18. Principle problem is the initial frequency of 186MHz - dividing that is beyond CMOS and 74HC(T) logic - you need a specialised prescaler.

A prescaler, such as a Motorola MC12013 (/10) or NXP SAB6456T (/64 or /256), followed by a divide-by-n counter 74HCT4059 would do that... note that at these frequencies constructional technique becomes critical (certainly for the prescaler) - a "Manhattan" technique would do the job...

The following circuit will take your divided pulse train and use that to fire a 555 wired as a monostable producing about 700uS pulses which get integrated by the 10uF (not 10pF as on the schematic) capacitor across the analogue meter (1mA FSD).

At 1kHz the pulses almost, but not entirely, join up which gives you the max frequency that this integrator will handle...

The reason you need the monostable is that your divided down 186MHz will have a 50:50 duty cycle, so its RMS value will be 50% of peak regardless of frequency, so you can't feed the divided down signal directly into an integrator.

However, the real problem is you only want a scale from 60MHz to 186MHz and you have an IF offset of 21MHz to remove as well. I have to say, this is classic micro-processor territory... it would be pretty trivial to do almost all this with a PIC or AVR processor for peanuts...

[AlanBeckett]

I thought there was a Link on here somewhere to a Chinese (?) Counter and Display PCB which allowed the IF Offset to be entered. However, I'm damned if I can find it

Alan

[pwdrive]

Forgot about the one shot but you get the general idea, maybe an Xtal marker would be easier if you can get your hands on a cheap Xtal, one transistor or maybe two, feed to RF in then mark the points on the tuning scale.

Lawrence.

[Nick]

I thought there was a Link on here somewhere to a Chinese (?) Counter and Display PCB which allowed the IF Offset to be entered. However, I'm damned if I can find it

These are very simple to do with a uP and normally limited to 40MHz as that's all CMOS handles - you still need the prescaler and the OP wants an analogue display.

What you need is, instead of driving an character LCD, for the uP to produce a PWM output proportional to the frequency range of interest - a simple RC integrator followed by a unity gain buffer & analogue meter would then be all that's needed.

The SAB6456T can divide by 64 or 256 and is designed to be driven directly from a local oscillator - 10mV sensitivity and accepts sine wave input - all the pulse shaping is done for you - the output of this could be fed directly into a uP which would then produce the adjusted PWM output. 2 chips only and a handful (maybe 10) other passive components, No CMOS divider, pots or 555 needed... you don't even need an xtal for the uP as its clock rate and stability is unimportant - you can use the internal oscillator so long as its fast enough to detect the edge trigger from the output of the prescaler.

EDIT: RATS! The SAB6456T seems to be discontinued - I loved this chip... However, there will be others out there that do the same...

[AlanBeckett]

Paul did say 'some kind of indicator', so not necesarilly an Analogue Meter.

Alan