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Re: spur & harmonics fix, v2

 

the 47 ohms resistors provide a resistive source impedance of appoximately 50 ohms. the emitter follower biased at 7 ma has an Re of 26/7 ~3 ohms. this 3 ohms and 47 os together provide a driving impedance of 50 ohms.
- f

On Tue, 20 Nov 2018, 21:11 iz oos <and2oosiz2@... wrote:

What is the purpose of the 47ohm resistors like R27 at the output of each rx and tx amplifiers? Are they really needed?


Il 12/nov/2018 09:01, "Ashhar Farhan" <farhanbox@...> ha scritto:
i have further simplified the fixes:

1. change the resistors R26 and R46 to 220 ohms (from 470 ohms). this increases the darlington pair's standing current and decreases the Harmonic distortions.

2. the 90 MHz trap was difficult to tune unless you had a spectrum analyzer. Instead, we can use a Low Pass Filter instead. A 'T', rather than a Pi was used. This helps better in isolation or RX/TX path.?

3. The LPF consists of an L-C-L of 300nh, 47pf, 300nh.?

4. With the relays that are now used for ubitx,? the harmonics at 3.5, 7 and 10 are in under -45 dbc. we don't need the axicom relays.

I am attaching a hand-drawn circuit to explain this better.

- f
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