W1DAN
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« Reply #26 on: March 11, 2025, 08:21:18 PM » |
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Hi All!
This easily can become a very deep subject as receiver design has advanced over the decades, from cat's whiskers to SDRs.
I enjoy the National HRO line of tube receivers, which have two RF stages. The earliest HRO had 6D6 tubes for the RF stages. This tube has a tranconductance of 1600 micro-mho's. The later HRO-60 had 6BA6's that have a transconductance of 4,400 micro-mhos. The dual-gate MOSFET that was used in many receivers in the 1970s has a transcunductance of 12,000 micro-mhos. There are related MOSFET transistors used today. Happily device gain has increased over the years, giving us more sensitive receivers with less overall noise.
Any amplifying device will have decreasing gain as the frequency is increased. This is due to interstage capacitance, usually grid or gate to plate or drain. When you have two RF stages in series, the loss at the higher frequencies will be double the loss of a single stage.
In the 1930s when the HRO was designed, mixer tubes were noisy, and signals were weaker (the majority of hams and SW broadcasters used less power). Combined with less knowledge or interest in receiver dynamic range, the natural thing to do back then was to amplify the antenna signal as much as possible before mixing to an IF frequency. This seemed good until you tuned off of a strong signal, where the AGC would bring the gain up and the front end and mixer would saturate.
If you look at the bandwidth of an RF front-end filter, it is not that narrow and will not reduce nearby strong in-band signals. Its main purpose back then was to reduce images (for a 455KC IF, the image would be 910KC away) and the HRO was good at this until about 14MHz (20M). Around 10M the images were only down about 20dB.
If using this topology it would be best here to have the first stage have low gain and low noise to provide some image rejection, then the second RF stage would do most of the amplification and have a good deal of dynamic range.
Today the better analog solution is to up-mix the HF range of interest to above 40MHz. A wide filter here gets rid of the images. Then this first IF signal is down-mixed to 9MHz or 455KHz where tight filtering can easily be provided. Of course, an SDR, being a direct-sampling receiver, has hopefully enough dynamic range, no images and the selectivity is created in math.
Ulrich Rohde N1UL, Wes Hayward W7ZOI, Doug DeMaw W1FB have excellent books on receiver design from the 1980s and 1990s.
Cheerio,
Dan
W1DAN
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