Eddystone 888A Field Strength Meter

[TrevorG3VLF]

I have just acquired an Eddystone 888A without an external field strength meter. Does anyone know what the circuitry was. Just a meter?

[G6Tanuki]

It's essentially a voltmeter (a 200uA microammeter with a series resistor) wired between the screen-grid and cathode of the second IF amp; there's a diode [one half of the noise-limiter double-diode) wired in series with the meter because under some settings of the IF gain control it would otherwise cause reverse current-flow through the meter.

[Skywave]

Originally Posted by TrevorG3VLF

I have just acquired an Eddystone 888A without an external field strength meter.

That meter - which was an optional extra - is not a field strength meter as such: it is a conventional S-meter. Right now there's one looking at me sitting next to my Eddystone 888A. The meter has an Octal plug on its cable and is intended to be inserted into one of the Octal sockets at the rear of the receiver. The other Octal socket provides an outlet for HT+ and 6.3 v.a.c. From memory, there's not a lot inside the housing of that S-meter: a pot. to adjust the electrical zero and a fat power resistor. But I've always been intrigued by Eddy's. calibration of 4 dB per S-point. Why 4 dB? 3 dB is double the signal power.

Al.

[Aub]

Isn't one S point equivalent to a 4 times increase in power = 6dB ?

Cheers

Aub

[Dual Standard]

Trevor,
try post #3 in this thread

https://www.vintage-radio.net/forum/...ad.php?t=47378

Mike

[Radio Wrangler]

Usually an S meter is set up to read the quiescent current of an AGC-controlled valve in the IF strip, which makes it an AGC meter. With stronger signals, the AGC line goes more negative and quiescent currents are backed off to reduce the gain of variable-mu stages.

This explains why either right-hand zero meters are used, or else normal left-hand zero meters are used embedded in a bridge circuit.

Many different values have been recommended for S meter intervals, but 6dB is good. S meter 'linearity' has long been a joke. Many have no semblance of accuracy. Some are halfway useful.

David

[Skywave]

The general idea of an 'S-meter' is to measure the signal strength of a received signal, although it is often used for comparative purposes. The following idea has occasionally crossed my mind.

With the aid of a calibrated signal generator, it should - in theory - be possible to use such a generator to calibrate your S-meter. Various controls of the receiver will need to be set to pre-determined settings, of course (such as IF gain, bandwidth, etc.), and for a constant r.f. input level, the meter reading will not only vary over given a tuning range but also from (switched) range to range - i.e. operating the bandswitch. However, if a set of calibration charts are made, they should result in S-meter readings that have some utility. In effect, the overall result is that the 'radio' plus S-meter becomes a tunable voltmeter - which thus extends the use of that combination to many other fields.

Just an idea.

Al.

[G6Tanuki]

IMHO inless a "S-meter" is properly calibrated [which generally means it's fitted to a measuring/test-set not a general-purpose radio-receiver, and you're feeding it a signal from a fixed-impedance non-reactive source...] the only real use for such a meter is for things like peaking-up an ATU or optimising the direction-of-point of a beam antenna.

[I once had to design a precision logarithmic detector with a 60dB operating range. Not an easy task, and it cost quite a bit more than your average amateur transceiver. So what hope for a S-meter in such a transceiver where the designer probably only had a couple of dollars-per-feature to spend?]

[Radio Wrangler]

The calibration of an S meter will probably also change if you can select different filter bandwidths.

Selective voltmeters were used for years monitoring and testing FDM multiplex phone trunks where they got called Selective Level Measuring Sets. All the usual culprits made them (HP3745A HP3746A etc)They usually have USB and LSB as well as a Demod output.
Because they're dedicated to measurement, the AGC steps in chunks, so it's better put to manual if you use one as a receiver. They're popular again for listening to LF/VLF signals.

There were also measuring receivers called Wavemeters as a sort of non-scanning non-crt hand-cranked spectrum analyser.

I'm afraid there are thousands of tons of boat anchors out there which beat you to it, Al.

David

[Skywave]

Originally Posted by Radio Wrangler

The calibration of an S meter will probably also change if you can select different filter bandwidths.

Which is a consideration that I have previously covered in my post #7.

Originally Posted by Radio Wrangler

There were also measuring receivers called wavemeters as a sort of non-scanning non-crt hand-cranked spectrum analyser.

I do recall - from many years ago - using an instrument which seems to match that description: an HFH, manufactured by R & S.
http://www.radiomuseum.org/r/rohde_f...eraet_hfh.html

Al.