The better transistors in the 45mhz IF should help diminish that 90mhz 2'd harmonic of the IF
that was causing the? (45mhz - DialFreq) spur on 15m,12m,10m.
For those curious about the math (arithmetic?) regarding this spur on v3 and v4 uBitx's:
The VFO (clk2) into the D1,D2 mixer is always above the 45mhz IF, so at 28mhz the VFO is a 45+28=73mhz
If the 45mhz IF into the D1,D2 mixer also has a 2*45mhz = 90mhz 2'd harmonic, that mixes with the VFO also.
So transmitting at 28mhz, we have a spur at 90mhz-73mhz = 2*45mhz - (45mhz+28mhz) = 45mhz-28mhz = 17mhz.
This only matters on 15m, 12m, and 10m, since on the lower bands the spur is above the transmit frequency
and can be taken out by the transmit low pass filter.? ?For 20m, the spur is at 45mhz-14mhz = 31mhz.
I'd expect this spur to not be so bothersome on the v5 as it was on the v3 and v4.
When transmitting CW, CLK2 is at the operating frequency and the 45mhz IF is quiet
(CLK0 and CLK1 are shut down).? So the spur only occurs when operating SSB.
Jerry, KE7ER
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On Wed, Feb 27, 2019 at 12:23 AM, Ashhar Farhan wrote:
There are two tricks :
1. BFR93W in the 45 Mhz IF amps, more overall gain.
2. A 470pf in the emitter degeneration of pre-driver instead of a 0.1uf gives more gain at 28 MHz than 3.5 MHz.
?
- f
