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From: Andy Talbot <andy.g4jnt@gmail.com>
Date: Sat, 14 Jul 2018 12:38:16 +0100
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To: LineOne <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: RE: Simple transverter for 472kHz
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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


Andy
www.g4jnt.com


On 14 July 2018 at 12:13, N1BUG <paul@n1bug.com> 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, jrusgrove@comcast.net 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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<div dir=3D"ltr">Those IMD figures don&#39;t in themselves say enough to sa=
y whether it is good or bad.<div><br></div><div>Firstly, that quoted paragr=
aph doesn&#39;t give the input levels at which IMPs were 60 [75dB] down.=C2=
=A0 =C2=A0It is more usual to give a third order intercept point=C2=A0 (TOI=
P)when specifiying linearity; a figure quoted in dBm</div><div><br></div><d=
iv>Secondly, specifying IMPs at different bandwidths is meaningless too, as=
 there is no indication of the receiver filtering.</div><div><br></div><div=
>So all in all, rejecting that receive converter based on that quoted parag=
raph is going too far.</div><div><br></div><div>If a level-7 (+7dBm Local O=
scillator) diode ring mixer were to be used at the front end, with no prece=
ding preamp and minimal loss filtering, you might reasonably expect a TOIP =
in the +15 to=C2=A0 +20dBm region.=C2=A0 =C2=A0That means IM Products will =
be, (in dB below theinput) twice the amount the RF input is below the TOIP.=
=C2=A0 An example :</div><div><br></div><div>TOIP =3D +20dBm</div><div>Two =
tone input at a level of -20dBm</div><div>IMPS=C2=A0 =C2=A0=3D=C2=A0 =C2=A0=
 +20dBm - 2 * (+20dBm - -20dBm =3D -60dBc on the input signals.=C2=A0 Or eq=
uivalent to -80dBm</div><div><br></div><div>So two -20dBm signal into a typ=
ical 7dBm LO diode ring mixer give -60dBc third order products.</div><div>A=
 diode ring is what any self respecting designer might use on a basic LF re=
ceiver converter</div><div><br></div><div>A level 13 mixer (+13dBm) wpouild=
 give a proprtionately higher TOIP, perhaps +25 to +30dBm</div><div>For hig=
her linearity / better strong signal handling still,=C2=A0 a bus-switch mix=
er offers a TOIP perhaps +35 to +45dBm.=C2=A0 =C2=A0Like the Softrock recei=
vers<br><div><br></div></div></div><div class=3D"gmail_extra"><br clear=3D"=
all"><div><div class=3D"gmail_signature" data-smartmail=3D"gmail_signature"=
><div dir=3D"ltr"><div><div dir=3D"ltr"><div><div dir=3D"ltr"><div><div dir=
=3D"ltr"><div><div dir=3D"ltr">Andy<div><span style=3D"font-size:12.8px"><a=
 href=3D"http://www.g4jnt.com" target=3D"_blank">www.g4jnt.com</a></span><b=
r></div><div><br></div></div></div></div></div></div></div></div></div></di=
v></div></div>
<br><div class=3D"gmail_quote">On 14 July 2018 at 12:13, N1BUG <span dir=3D=
"ltr">&lt;<a href=3D"mailto:paul@n1bug.com" target=3D"_blank">paul@n1bug.co=
m</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margi=
n:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Ouch! Having oper=
ated on 160m and the HF bands for several years<br>
using a receiver that comes in at about -63 dB on 2 kHz spacing<br>
third-order IMD I would never again buy something with that kind of<br>
receiver &quot;performance&quot;.<br>
<br>
Paul N1BUG<br>
<br>
<br>
On 07/06/2018 09:45 PM, <a href=3D"mailto:jrusgrove@comcast.net">jrusgrove@=
comcast.net</a> wrote:<br>
&gt; Saw the writeup in QST and immediately noticed the receive<br>
&gt; two-tone, third-order IMD at 2 kHz spacing is 60 dB and at 75 kHz<br>
&gt; spacing is only 74 dB! Good luck with that if you&#39;ve got strong<br=
>
&gt; signals at your location.<br>
&gt; <br>
&gt; Jay W1VD=C2=A0 WD2XNS=C2=A0 WE2XGR/2<br>
<br>
</blockquote></div><br></div>

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