Re: ubitx - stop press

selfy.dtp@...
 

Thank you Ashhar,

My 'observations' show that proper termination of the QER filter (and matching the crystals!) provides lower ripple in the pass-band together with lower losses. It really depends on what kind of crystals one is using. Mine required 150pF capacitors to ground to achieve 2.7kHz -3db pass-band and -4 dB loss @ 50 ohms at both ends.

My experience is limited only to Minima crystal filter. I modified the Rx part of the bidirectional amplifier to the original Wes/Kopski circuit with a gain of +24dB (theoretical). I am still fighting with the KISS mixer, but I guess I go into too much off topic here.

Thanks once again and 73!


On Sat, Apr 8, 2017 at 6:32 PM, Ashhar Farhan <farhanbox@...> wrote:
i have measurer the input impedance of these stages to be appox 50 ohms when terminated in the 220 resistor in the output. i made these measurements with a homemade return loss bridge and the sweeperino. 
the original wes/kopski amps uses separate resistors for bias and feeback, i used just one. if you want to experiment with different feedback resistors while keeping the bias the same, you could use the original block.
the noise figure should be 14 db. i say that because i havent measured it. i do not have calibrated noise source. this figure is based on the accumulated (measured) losses of the lpf, first mixer and the first if amps figure of 6 db.
connecting the antenna brings up the noise on all bands at my qth. i must, though admit that i havent had any contacts in 28 mhz at all.
- f

On 8 Apr 2017 7:52 p.m., <selfy.dtp@...> wrote:
Farhan,

Let me first thank you for the projects and experience you share with us. I am interested to find out how uBITX behaves in terms of selectivity and sensitivity?

I also support the idea, that if you want to cover all HF band a set of (at least) four LPF for the PA is a must.

Additionally, looking at the uBITX schematic, I couldn't help but notice the bidirectional amps slightly differ form the original idea of W7ZOI and K3NHI. You used different biasing approach, but it appears the input impedance is not 50 ohms. My LTSpice simulations show 90 ohms at 10MHz and 50 ohms at 45MHz. And the "gain" transistor is biased at 15mA. Would you, please, share with us what your idea was for doing these mods.

73 de LZ1NEF



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