Re: Test for solving Spurs #ubitx

Jerry Gaffke

My understanding is that these spurs arise primarily in the mixer, not in the 45mhz IF amp itself.
First off, the Si5351 with the 6dB pad is not giving anywhere near 7dBm into the mixer LO port, more like 0dBm.
The 45mhz signal entering the mixer should be 10dB below the local oscillator, so roughly -10dBm,
anything more than that and the mixer starts misbehaving.   
These are some very round numbers, but should give an idea.

Gain distribution in the transmitter is such that we can have too much 45mhz signal entering the first mixer,
even if we did have 7dBm at the LO port.
Especially true if you crank up the audio to get maximum power out to the antenna.

Farhan mentioned ALC as a possible solution to this.
Seems simpler to design the 45mhz amp to give constant gain regardless the actual performance of the
transistor in that particular rig, this means we need e a transistor with a higher Ft than the 2n3904.
Also, a procedure for measuring the maximum audio signal out of the mike amp, a DVM set to AC volts 
might be sufficient.

If ALC is desired, John VK2ETA's scheme of moving the 45mhz IF signal around
within the 45mhz filter passband is a zero additional hardware solution that gives good results.
Allison also had a solution involving 1n4007 pin diodes in the emitters of the IF amps,
a $0.25 hack.
But ALC for transmit seems an unnecessary complication for a simple rig like the uBitx.

Jerry, KE7ER


On Sat, Aug 11, 2018 at 08:49 AM, Henning Weddig wrote:


I was wondering why the first IF could cause problems! But due to Your explanation I now understand the problem! It is not caused by the If filter itself but in TX mode by the amp beween the 45 MHz filter and the mixer, producing a second harmonic and even higher harmonics if driven too hard. So the effect must be output power related. As the transistors within this amp are limited in gain  due to their limited gain bandwidth product (my LTSPICE simulation showed a 3 dB gain reduction) higher harmonics must be much lower.  Using e.g. BFR106 insteead could worsen the spur problem.

A simple remedy could either be a lowpass filter between the output of the 45 MHz TX am and the mixer input or simply a 90 MHz trap (bandstop filter)? Maybe a 45 MHz LC bandpass filter also could help.

  Henning Weddig


Join to automatically receive all group messages.