[ARC5] Receiver Sensitivity - Power Supplies

[Brian Clarke]

My error Bill,

 

Only the R-23, R-23A and R-24 and their SCR-274N equivalents have adjustable
sensitivity / bandwidth by altering the spacing of the coils in the IFTs.
The ARC-5 Alignment handbook says for these receivers the IFT coils should
be loosely coupled = narrower bandwidth. It certainly makes alignment
easier.

 

Using 'just discernible signal' is not the standard way to measure
sensitivity - it presents inter-subjectivity reliability problems. The usual
sensitivity test, paraphrasing Robert's offering, is to apply a modulating
tone to a carrier, lower the sig gen output till further lowering has no
effect on received audio level, and then switch off the modulation. Measure
the difference in audio output level between the modulation on and off
states. You may have to raise the carrier level to get one of the standards
- 10 dB difference.

 

I use an HP 8640B as my sig gen, too, with an inline coaxial 20 dB
attenuator, and lock the frequency. The military version of the 8640B, the
USM-323 doesn't have this facility and the frequency drifts enough to be a
pain when you're trying to align for best performance. At first, I thought
this was a problem with the 323 until I tried the 8640B unlocked - exactly
the same drift rate. So, either be quick or, knowing the direction and speed
of drift, start some 'distance' away and measure when the frequency readout
is exactly what you want. At these low levels, I'm sure you know about using
short, double-braided coaxial leads, like RG-214, or if you're not concerned
about impedance, quad-shielded RG-6, the TV installer's friend. There's a
huge imbalance between the 8640B's output impedance the Command Rx antenna
input impedance - so, 50 Ohm or 75 Ohm cable will have negligible effect.

 

As an aside, I was measuring the noise level of a DC-coupled tube audio
amplifier I have designed and built. As well as using a very good Marconi
signal level meter, I was listening on my Beyer DT-480 monitoring cans and I
could hear this very low level, low frequency noise. Then, I stopped
gritting my teeth and the noise disappeared - I was hearing my own blood
flow. This was at -129 dB re 30 W, = 3.8 x 10^-12 W, or 3.8 pW.

 

73 de Brian, VK2GCE.

 

On Wednesday, 2 May 2018 4:01 PM, you said:

 

Dear Brian,

I'm going to have to review the manual's alignment instructions - I don't
recall if there was an option for (I.F.) BW or selectivity. I do remember
checking my AC voltmeter very carefully against a reference, as this is what
I used to measure output across a 600 ohm resistor. (I seem to recall making
up a temporary "jig" to make this process a little easier...)

 

The minimum discernible signal was much lower, but the signal generator only
goes down to 0.1 microvolt. I've since acquired some coaxial attenuators
which I could attach to the front of the signal generator for really "low"
signals... (Since that time, the "original" HP 8640B quit operating. I have
a replacement on long-term "loan" from a friend. He bought a couple, and
this was given to him because it was "dropped" and has visible damage. He
and I tested it, and it still works, so I'm using it.Who cares if it's
missing the cast aluminum handle on the right side?)(And some of the plastic
knobs are broken...)

 

Bill KA3AIS

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180503/56e8fd42/attachment-0001.html>