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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: Sun, 15 Jul 2018 07:50:05 +0100
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Yes, I agree with your figures and they are terrible.  -125dBm MDS means -128dBm of noise in a CW (call it 300Hz) bandwidth,  that means a noise figure of around 21dB so there is no front end gain, and almost certainly some loss before mixer. 

So a -36dBm IP3 is appalling.   The increases to -14dBm at 75kHz just means there is some filtering in place.

That low an IP3 is the sort of figure you'd expect, perhaps, from a single bipolar mixer;  the sort of thing used in the 1960s and 1970s (as I did in my first homebrew HF receiver !)

Could there have been something wrong with the test unit perhaps?   LO power drastically reduced into a mixer?  It would be difficult to make a mixer with that low an IP3  othewise


On 15 July 2018 at 00:54, [email protected] <[email protected]> wrote:
Andy
 
Maybe I can shed further light on my comments ... 
 
The ARRL receiver measurement system was developed by Wes Hayward of Tektronics back in the mid 70's and has been used with minor changes since. Instead of calculating 3IP the system quotes third order IMD dynamic range relative to the noise floor (MDS - minimum descernable signal). The MDS is deemed as the signal level at which a 3 dB increase over 'background' noise is noted - as measured by an audio voltmeter.  The measurement is normally made with the receiver in a cw bandwidth and is quoted in dBm. QST quotes the MDS for the this transverter as -125 dBm. For the two-tone IMD dynamic range measurement, two tones (at selected spacings) are injected into the receiver and the level is increased until the third order products are 3 dB above the background noise - at the MDS level. The two-tone dynamic range is the difference between MDS and the signal generator level.
 
For this transverter with a -125 dBm MDS and 60 dB dynamic range at 2 kHz tone spacing, the indicated signal level to cause third order IMD products would be -65 dBm. At 75 kHz spacing the signal level would be -51 dBm. Calculating IP3 for 2 kHz spacing using -125 dBm and -65 dBm produces an IP3 of -35 dBm. Calculating IP3 for 75 kHz spacing using -125 dBm and -51 dBm produces an IP3 of -14 dBm.
 
I normally think in dynamic range numbers, not IP3, since I've been using the 'Hayward system' since the beginning. Since you think in IP3 numbers how do those IP3's look to you?
 
Jay W1VD  WD2XNS  WE2XGR/2      
 
 
----- Original Message -----
From: Andy Talbot <[email protected]>
Reply-To: <[email protected]>
To: LineOne <[email protected]>
Sent: 7/14/2018 7:38:16 AM
Subject: Re: LF: RE: Simple transverter for 472kHz

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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