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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