Re: Tracing receive signal for low signal

Jerry Gaffke
 

That's a pretty good summary, here's some minor corrections and
a major dump of nitty-gritty detail:

>  If you can fix the harmonics, then operation on bands 20m and lower in frequency 
> should not have the IMD issue.  You can reduce that with both Ashhar's and Raj's fixes combined.

Transmitter harmonics occur at integer multiples of the transmit frequency.
So if transmitting at 7mhz, you might have harmonics at 14 and 21mhz.
I would be happy enough on the harmonics  if folks just replaced KT1,KT2,KT3
with proper RF relays such as the Axicom, flat on the board.

Spurs are similar to harmonics, except they occur at unexpected frequencies that are not
multiples of the transmit frequency, can be due to harmonics in an earlier stage of the rig.
For example, the 90mhz harmonic of the 45mhz desired signal in the 45mhz IF stage
will also mix with the high side clk2 VFO, producing a spur at 2*45mhz - (DialFreq+45mhz)
Or your transmitter output (perhaps 7mhz) sneaks back into the 45mhz IF, and that mises with the VFO.
Raj and Farhan have both suggested fixes that are apparently good enough on the spurs
(replace L5,L7 with special shielded inductors, and/or add a low pass filter going into
the mixer at D1,D2 to knock out the harmonic from the 45mhz IF amp at 90mhz, respectvely).
One or the other (or both) should bring spurs mostly into spec.  (Might also want Farhan's 12mhz trap 
to deal with 12mhz crosstalk from clk0 into clk2, creating a 12mhz spur).
The worst spurs are at 15m, 12m and 10m, though there can be spurs beyond the -43dB spec on
the lower bands too.

IMD (intermodulation distortion) is what happens when a band of frequencies goes through a non-linear amp.
Those various components of your signal interact with each other, creating new close in frequencies.
For example, you are transmitting a two tone USB test on 7.2mhz, the tones at 7.2010 and 7.2015 mhz,
The second harmonic of 7.2015 at 14.4030 mixes with 7.2010 producing a new tone at 7.2020.
On the uBitx, IMD is mostly due to having the mike audio higher than it should be, the diode ring mixers
and the two IF amps get more signal than they can cleanly deal with and thus become non-linear.
Also some IMD due to the final amp.  Fixing IMD would take a major redesign of the uBitx, redistributing
gain away from those IF amps and adding more gain to the final power amp, nobody has given a
step-by-step recipe to do this yet in the forum.  Fortunately, IMD is considered a lesser sin than 
spurs and harmonics, the extra cruft is so very close in with the main signal such that everything still
pretty much fits into the bandwidth of a legal AM signal.

An RTL-SDR using one of the DVB-T dongles uses an 8 bit ADC to digitize the entire world of incoming signals.
There's generally enough noise to see only 7 valid bits of data out of such an 8 bit ADC.
An increase in signal power by 4x is equivalent to +6dB, and since power into a fixed 50 ohm load 
is proportional to the square of the voltage, that same +6dB is a doubling in the signal voltage. 
Each of those 7 binary data bits represents a doubling of voltage, so teh RTL-SDR has a range of 7bits*6dB/bit = 42 dB.
Thus an RTL-SDR might be able to simultaneously see both your desired signal and a spur (or harmonic) that is 43dB down,
but you will have to set up your step attenuator just right for them both to fit into the 7 bits.  Trying to measure a spur that
is 50 dB down with a device using an 8 bit ADC is totally doomed.  However, an RTL-SDR is good enough to catch
the really bad cases, there have been reports of uBitx spurs and harmonics that are only 30dB down.

Asking a friend to check for spurs and harmonics is unlikely to be very thorough.
The emissions will vary with each band you use, and to some degree as you move across the band.
The spurs can be most anywhere, your friend would have to scan the entire HF spectrum.
This sort of check is best carried out with a spectrum analyzer,
using a communications receiver will be very time consuming if you want to do a good job of it.

I'd recommend at least Raj's fix of replacing L5,L7 with his specific shielded inductors (easy!)
and replace the relays at KT1,KT2,KT3 with axicom, no sockets.  (Tough to remove.)
Alternately, instead of replacing the relays, could use an external low pass filter appropriate for each band
in line with the antenna.   Do not goose the mike gain to get more power out.
Then the rig is mostly compliant with the US -43dBc spec

I haven't heard any reports of any of these rigs attracting legal attention, even without mods..
But I would not consider using a uBitx with an external linear amp without an awful lot of work
on the uBitx first.

Jerry, KE7ER



On Tue, Jan 15, 2019 at 06:25 AM, Evan Hand wrote:
I go the relays at Mouser:
https://www.mouser.com/ProductDetail/te-connectivity/v23105a5403a201/?qs=Nk6ydC%2fJI6we9mx5p4DWfg%3d%3d&countrycode=US&currencycode=USD

I believe you can do better on price if you go through ebay, though the lowest cost may take awhile in shipment.

I am not sure that I would trust my RTL-SDR to accurately measure for the harmonics/spurs.  I got different results when compared to my RF Explorer WSUB1G+, and the front end of the RTL-SDR seems to overload easily, giving strange results.  Again, I am not sure which one to believe.  They both do agree that there is a 3rd order harmonic at 21mhz that is higher than the -43db that is required in the US.

I would also read the following threads:
https://groups.io/g/BITX20/message/61256
https://groups.io/g/BITX20/topic/27001748#61457
https://groups.io/g/BITX20/topic/28049653#61551

There are more and the net of all of this is that there is not total agreement on the fixes that are required.

My interpretation is as follow (please understand that I am NOT and expert.  I have just read a LOT of the emails related to the uBitx)
1 - The relay replace is minimum and may not totally fix the harmonic issues.  The true fix is an external low pass filter board linked to the uBitx switching relays, or possible loss in receiver sensitivity if left in and manually switched when the band is changed.

2 - If you can fix the harmonics, then operation on bands 20m and lower in frequency  should not have the IMD issue.  You can reduce that with both Ashhar's and Raj's fixes combined.

3 - without the major modifications that are suggested by the v5 board changes, the v4 and earlier should NOT be used as the exciter to a PA, UNLESS you have the equipment and skill to verify the spurious radiation is under control.

I have been told in this group that in the "old days" spurious emissions where tested using another receiver located far enough away to not overload the front end.  In other words, have a friend that can hear your signal verify no spurious emissions.  This would involve scanning for them.  I am sure that was state of the art then, not sure the FCC would agree now.  We are responsible for our transmissions.

It has also been pointed out that with the normal power levels of the uBitx, that as long as you were close, the spurs would be so far down that it would not really cause interference or be noticed.  One of the reasons for me abandoning the PA until I know what to do to the uBitx.

As always, take my input with a grain of salt.
73
Evan
AC9TU

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