SECTION 5 — Main receiver with 2 outputs
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I crafted a simple DC receiver. At some point, I might design and build a phasing ( ZIF receiver ) to supplant this module.
Above — I placed addition low-pass filtration on the input to allow receiver use with lesser grade preamp/filters than shown in Section 2 — and to scrub off any remaining VHF-UHF garbage at the input. I chose an SBL-1 mixer from MiniCircuits labs because I’ve got many left over from an estate sale purchase many years ago.
Above — Simulation of the ideal diplexer using high Q caps + inductors. In reality, the L & C parts aren’t high Q and performance will fall short of ideal. But considering its parts count and simplicity, this diplexer seems practical.
Above —GPLA simulation of the input low-pass filter. It delivers a good input match to the RF port of the diode ring mixer all the way up to 26.4 MHz
I spent much bench time on the feedback amp [ FBA ]. With a AF return loss bridge I tweaked the feedback and current to realize a input return loss of 25.8 dB at 1 KHz. The low noise
BC337 for Q2 and Q3 adds very little noise to the Johnson noise incurred in the bias + feedback resistors. Measured gain of the feedback pair = 28 dB with only 11.6 mA current. I designed a lower noise version, although current draw was over 35 mA so opted for the FBA shown.
A ripple filter + careful AF bypass & RF bypass removed any hum and RF from the output.
Above — Audio amp and splitter schematic. 2 LM4562 op-amps make up the virtual ground plus 3 stages of low-pass filtered amplification. Measured gain = 24 dB. A Butterworth response employing low Q filter poles helps keeps ringing down. Low value resistors throughout keep Johnson noise down.
U3 serves as an active splitter/buffer to provide low impedance output to 2 panel mounted RCA jacks. The output 22K resistor is normally in parallel with the load and serves to discharge the 3.3 µF caps when its port is unplugged.
Above — I connected the receiver [ with cover off ] to some bench modules ( left to right ) AF power amp + speaker; homebrew VFO/signal source; 7 MHz band-pass filter to coax and an 1/4 wave 40M band vertical antenna with 43 radials.
The band conditions were good and I listened to this receiver for 3 nights. Pleased, I boxed it up and moved onto the next module.