Re: Open HPSDR Hermes (14 bit ADC, EP3C25) Transceiver Card

Conrad, PA5Y

My measurements are from a K3S and TS-890S, with a 2 tone signal at 0dBm from the transverter port. The difference between the 2 fundamental tones is better than 50dB down at the 3rd orders, I will measure it again tonight and post it to you.


Where do you get the idea that a class A RF stage is incapable of such performance? Your -40dBc for Class A is typical for RF power amplifiers running at tens of watts.


Look here at figure 3a, this is a Class A RF amplifier with no pre-distortion running at 2W per tone. The IM3 is -51.48dBc, this is at 950 MHz.


When testing the 32S-3 Rob is testing the audio chain IMD as well as the finals. To properly measure an amplifier IMD you should use 2 isolated sources.


This is not a bad read.


I am not saying that I can achieve his level of performance at my final tetrode PA, just at the transverter input and the transverter itself.




Conrad PA5Y




From: <> On Behalf Of oldjackbob via
Sent: 25 February 2022 15:42
Subject: Re: [SDR-Radio] Open HPSDR Hermes (14 bit ADC, EP3C25) Transceiver Card


On Fri, Feb 25, 2022 at 04:03 AM, Conrad, PA5Y wrote:

The 0dBm from either my K3S or TS-890S is very good indeed. IMD3 from both is better than -50dBc.


I don't understand your numbers at all. A class A amplifier is capable of producing better IMD3 numbers than any other amp, and it is only capable of ~40dBc IMD3.

Rob Sherwood says the Collins 32S-3 is the best transmitter he has ever tested (other than an ANAN) and he compares all modern transmitters to the 32S-3. The numbers he measures on that transmitter are only -42dB referenced to PEP (see slide #29 in this pdf):

Rob has stated that "Except for predistortion (PureSignal) available from Apache and Warren Pratt, and a couple of Yaesu class A rigs (if zero ALC and processing is run), all transmitters today are worse than a Collins 32S-3 as far as transmit IMD.":

So how on earth are you getting "better than -50dBc" IMD3 numbers on the 890S and K3S?


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