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G6Tanuki · 28th Jan 2014, 12:12 pm

From the traditional valved HF communications-receiver perspective the "two RF-amps" thing had several purposes:

[1] To allow three loosely-coupled RF tuned-circuits ahead of the mixer. This helped reduce image response [which despite 3 tuned-circuits was still a problem at 25+ MHz if your IF was only 465KHz]

[2] To overcome mixer-noise [old multi-electrode mixers like the ECH35 and 6SA7 had equivalent-noise-resistances of 250,000 Ohms-plus and you needed significant RF amplification to get to a point where first-RF-tuned-circuit noise overcame mixer-noise]

[3] An extension of [1] - the additional pre-mixer selectivity could give reduced cross-modulation effects in the mixer [but paradoxically the extra pre-mixer amplification could increase the risk of it!]

[4] Pre-mixer selectivity provided greater isolation of the LO signal from the antenna - potentially important in some military/commercial applications where multiple receivers were fed from the same antenna [or if you were at risk of the enemy DFing your LO signal].

In the context of FET or bipolar VHF/UHF TV tuners, image interference [where extra tuned-circuits ahead of the mixer would be a Good Thing] never seemed to be a big issue - not in the UK anyway. Bands IV and V had national/regional station-planning so that you rarely had a significantly-powerful local transmitter on the image-frequency of another station serving your locality: though I must admit to having experienced image-reception of aircraft on a Band-II receiver.

I can see the logic in having two MOSFET RF amps in a broadcast-band tuner: the first one essentially being noise-figure-optimised [with the AGC delayed quite a bit], the second one being deliberately designed to provide relatively low-gain (tap the gate-connectors well down the tuned-circuits so you get the highest possible tuned-circuit 'Q') 'selective-matching device with a bit of gain' then into the mixer.

I must dig out some of the old diagrams and suchlike that I've got which relate to 1960s-vintage American PMR gear: the likes of GE and Motorola knew their stuff when it came to providing utterly bomb-proof-but sensitive VHF receiver front-ends and used circuitry which looks quite alien [4-chamber helical filters for example] when compared to something like a Pye Westminster [which used ordinary slug-tuned coils and a humble 2N3819 in the first RF-amp].

28th Jan 2014, 12:12 pm	#80
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: FET Questions From the traditional valved HF communications-receiver perspective the "two RF-amps" thing had several purposes: [1] To allow three loosely-coupled RF tuned-circuits ahead of the mixer. This helped reduce image response [which despite 3 tuned-circuits was still a problem at 25+ MHz if your IF was only 465KHz] [2] To overcome mixer-noise [old multi-electrode mixers like the ECH35 and 6SA7 had equivalent-noise-resistances of 250,000 Ohms-plus and you needed significant RF amplification to get to a point where first-RF-tuned-circuit noise overcame mixer-noise] [3] An extension of [1] - the additional pre-mixer selectivity could give reduced cross-modulation effects in the mixer [but paradoxically the extra pre-mixer amplification could increase the risk of it!] [4] Pre-mixer selectivity provided greater isolation of the LO signal from the antenna - potentially important in some military/commercial applications where multiple receivers were fed from the same antenna [or if you were at risk of the enemy DFing your LO signal]. In the context of FET or bipolar VHF/UHF TV tuners, image interference [where extra tuned-circuits ahead of the mixer would be a Good Thing] never seemed to be a big issue - not in the UK anyway. Bands IV and V had national/regional station-planning so that you rarely had a significantly-powerful local transmitter on the image-frequency of another station serving your locality: though I must admit to having experienced image-reception of aircraft on a Band-II receiver. I can see the logic in having two MOSFET RF amps in a broadcast-band tuner: the first one essentially being noise-figure-optimised [with the AGC delayed quite a bit], the second one being deliberately designed to provide relatively low-gain (tap the gate-connectors well down the tuned-circuits so you get the highest possible tuned-circuit 'Q') 'selective-matching device with a bit of gain' then into the mixer. I must dig out some of the old diagrams and suchlike that I've got which relate to 1960s-vintage American PMR gear: the likes of GE and Motorola knew their stuff when it came to providing utterly bomb-proof-but sensitive VHF receiver front-ends and used circuitry which looks quite alien [4-chamber helical filters for example] when compared to something like a Pye Westminster [which used ordinary slug-tuned coils and a humble 2N3819 in the first RF-amp].