Many S-meters
are useless, here is shown modified S-meter from Atlas 210-X. It was 1dB pr
S-unit from S1-S7, in fact only one actual S-unit! D1 is used to define the
threshold, and the combination of a diode a resistor (D2/R3) reduce
deflection when the meter just deflects. In the actual case it was
possible to achieve a reading of 4-5dB per s-unit. D1= 1N34A, D2=1N4148,
transistor could be a discrete device like BC547 or 2N2222, but for Atlas
210X it is part of an integrated circuit.
Reference: Present-day receivers - some cures and
cures, Ham Radio dec.1977 pp10-18 (W0JGP, K8RRH) page 17: The Atlas
210X without its noise blanker operational, has a better than average
dynamic range of about 90dB, which would be even better if its
double-balanced mixer were properly terminated above the IF frequency (*).
This could be accomplished with the use of a diplexer, as descrided by Wes
Hayward or with a power jfet, as related by U L Rohde. There is one
limitation in the 210X that cannot be easily remedied, however; its
potential strong-signal handling capabilities cannot be fully
realized due to its noisy conversion oscillator. Since this oscillator has
noise sidebands that are only 65dB down 10kHz on each side of its center
frequency, all signals passing through the mixer will take similar noise
sidebands. Consider a strong station near a desired signal that is weaker in
amplitude. Reciprocal mixing of oscillator and noise can cause noise
sidebands to be transferred to the strong nearby station and cause interference
to the desired signal. Thus even if the i-f filter's ultimate rejection is
actually realized in the receiver circuitry, which is doubtful in practice,
this high level of rejection can be negated by wide-band oscillator noise.
So while it takes two strong signals to cause IMD which can interfere with
weak signal reception, a noisy oscillator and a strong signal can cause the
same unfortunate results.
The noise blanker in AStlas 210X also degrades
its dynamic range, diminishing the advantage of the double-balanced passive
mixer. The 210X transceivers we tested had a dynamic range of between 73 and
81dB, depending on the boand selected. When the blanker was turned on, these
numbers dropped by 3dB
Note*) Ulrich Rohde DJ2LR, "High dynamic range
receiver input stages" ham radio, october 1975, page 26.
