Correction: 

I wrote:
>So noise figure is 21dB.

It should have read 22dB.

73
Clemens
DL4RAJ 

>-----Original Message-----
>From: [email protected] 
>[mailto:[email protected]] On Behalf Of Clemens Paul
>Sent: Sunday, July 15, 2018 10:43 PM
>To: [email protected]
>Subject: RE: LF: RE: Simple transverter for 472kHz
>
>Hello Andy,
>
>>Yes, I agree with your figures and they are terrible.  -125dBm 
>>MDS means -128dBm of noise in a CW (call it 300Hz) bandwidth, 
>
>-125MDS means that noise is at -125dBm,not at -128dBm.
>You feed a signal of a sig gen into the receiver and adjust 
>its level until a RMS AF voltmeter at the receiver's
>AF output shows an increase of 3dB compared to no signal at input.
>This means that signal and noise have the same level, hence 
>the increase of noise+signal by 3dB.
>The dBm readout of the sig gen then is MDS.
>ARRL use 500Hz as 'CW-BW'.
>So noise figure is 21dB.
>According to CCIR curves man made noise in a "quiet receiving 
>site" is 60dB.
>Probably today even higher.
>So there is plenty of room to shift the 
>transverter's/receiver's dynamic range up by additional input 
>attenuation
>if it should be necessary to avoid 3rd order IM.
>The front end topology can be found in the manual and is:
>500kHz 7-pole LPF => 475kHz roofing filter with ceramic 
>elements => 5-pole 500kHz LPF => mixer 
>
>>Could there have been something wrong with the test unit 
>>perhaps?   
>
>...or with the test setup at such a low frequency?
>
>73
>Clemens
>DL4RAJ 
>
>
> 
>
>>-----Original Message-----
>>From: [email protected] 
>>[mailto:[email protected]] On Behalf Of Andy Talbot
>>Sent: Sunday, July 15, 2018 8:50 AM
>>To: LineOne
>>Subject: Re: LF: RE: Simple transverter for 472kHz
>>
>>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
>>
>>
>>Andy
>>www.g4jnt.com
>>
>>
>>
>>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
>>              
>>
>>
>>              Andy 
>>              www.g4jnt.com <http://www.g4jnt.com/> 
>>              
>>
>>
>>              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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