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Re: LF: RE: Simple transverter for 472kHz

To: LineOne <[email protected]>
Subject: Re: LF: RE: Simple transverter for 472kHz
From: Andy Talbot <[email protected]>
Date: Sat, 14 Jul 2018 12:38:16 +0100
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Those IMD figures don't in themselves say enough to say whether it is good or bad.

Firstly, that quoted paragraph doesn't give the input levels at which IMPs were 60 [75dB] down.   It is more usual to give a third order intercept point  (TOIP)when specifiying linearity; a figure quoted in dBm

Secondly, specifying IMPs at different bandwidths is meaningless too, as there is no indication of the receiver filtering.

So all in all, rejecting that receive converter based on that quoted paragraph is going too far.

If a level-7 (+7dBm Local Oscillator) diode ring mixer were to be used at the front end, with no preceding preamp and minimal loss filtering, you might reasonably expect a TOIP in the +15 to  +20dBm region.   That means IM Products will be, (in dB below theinput) twice the amount the RF input is below the TOIP.  An example :

TOIP = +20dBm
Two tone input at a level of -20dBm
IMPS   =    +20dBm - 2 * (+20dBm - -20dBm = -60dBc on the input signals.  Or equivalent to -80dBm

So two -20dBm signal into a typical 7dBm LO diode ring mixer give -60dBc third order products.
A diode ring is what any self respecting designer might use on a basic LF receiver converter

A level 13 mixer (+13dBm) wpouild give a proprtionately higher TOIP, perhaps +25 to +30dBm
For higher linearity / better strong signal handling still,  a bus-switch mixer offers a TOIP perhaps +35 to +45dBm.   Like the Softrock receivers

On 14 July 2018 at 12:13, N1BUG <[email protected]> wrote:
Ouch! Having operated on 160m and the HF bands for several years
using a receiver that comes in at about -63 dB on 2 kHz spacing
third-order IMD I would never again buy something with that kind of
receiver "performance".

Paul N1BUG

On 07/06/2018 09:45 PM, [email protected] wrote:
> Saw the writeup in QST and immediately noticed the receive
> two-tone, third-order IMD at 2 kHz spacing is 60 dB and at 75 kHz
> spacing is only 74 dB! Good luck with that if you've got strong
> signals at your location.
> Jay W1VD  WD2XNS  WE2XGR/2

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