RFP-1: The Test & Evaluation
Home
Next
Back
I've run all the requisite bench tests to confirm
the utility of the circuit design. But as
necessary as all that is, to confirm the circuit,
parts selection, architecture, and wiring, I (as
always) was chomping at the coax to get this
thing on-the-air.

As all good radio-persons know; the proof of
the pudding in the tuning ... or something like
that.

First, the MMIC preamplifier chip is quiet.
There is no perceivable added noise, although
on the bench, the test equipment indicated
what Minicircuits advertised their MAR-6A to
do: about 20 dB and an NF of about 3 to 3.5
dB. I have used 500K linear pots for the
"Drive" (attenuator) and "Gain" controls. They
put the source and load impedances well
within the capabilities of the preselector's L/C
filters. I chose 10:1 so as to not load down the
thru-bus circuitry (50 Ohms) and yet allow
enough rotational control over the indicated
functions.

I have discussed the precelector functions
elsewhere on this site. To recap, the "Q" of
the series, low impedance, L/C combinations
is certainly high enough to provide a high
degree of on-channel filtering. Depending on
where it's employed, it will provide about 18 to
48 dB of selectivity -- that's the actual
bandpass for the selected frequency or
band-stop for interfering adjacent stations.

The 4 "Level" controls are actual RF Gain
controls and fed from the 4-way wideband
multicoupler. The advantage to these is that
the operator can control the various receivers'
input signal level prior to the signal entering
the receiver. This is an important
consideration in controlling the radio's front
end and AGC functions, thereby reducing or
(actually) eliminating intermod and overload.
For convenience, each level control has a
switch which disconnects, and grounds, the
radio's connection -- for muting purposes.
Additionally, the operator does not have to
readjust the control when returning to that
particular radio.

For this project, I elected to use transformer
matching at the front end of the RFP-1. On
other models, I used Pi-Netork tuners, but
have come to appreciate the utility of a single
knob for matching to the most appropriate
"neighborhood" of feed impedance, rather
than fine tuning 3 knobs for an exact match.
Regardless of what you have heard,
impedance matching is important for receivers
if hearing very weak signals is important to
you. Since matching the antenna at the
desired frequency increases the energy
transfer at that frequency, it tends to reduce
out of band noise, which is coupled in a less
efficient manner. This may seem like a
somewhat insignificant issue for most
surveillance and intercept requirements, but
the effect can be quite noticeable during
difficult monitoring tasks.

The VLF Converter works very well. It is
actually a 10 MHz crystal upconverter. Tuning
from 20 KHz to 500 KHz is accomplished on
the receiver's 10 MHz band -- from 10.020 to
10.500 MHz. It is clean and accurate, and
actually has some gain.

The Marker Generator is designed for 100
KHz. There's not much to say about that and
it's probably a superficial option given today's
radios, but it sure is nice to have when using a
Collins 51S1, R390, or other analog display
radio.
The green LED indicates the particular band selected of the
Selectivity section of the preselector. High-Q or wider Low-Q can
be selected (and is indicated by the brightness of the LED).
The blue LEDs indicate that all bands are available and that
adjustment is broad-banded through the use of the "Trim" control
in the Selectivity section of the preselector.
The red LED indicates that the preselector function is bypassed
and that no bands may use any Selectivity adjustments. Switching
from green to red (or blue to red) quickly allows the operator to
hear the reception effects of the preselector.
Counter