HI,
>>>For holiday fun and learning I'm homebrewing a uBITX architecture radio but trying to upgrade it a bit along the way by being able to swap the TIAs with SBL-1/2N5109 mixer/post amps ala the classic Progressive receiver (both because I have boards to do that and in hopes of a bit more robust receiver) and adding a HYCAS IF amp.<<<<
By that do you many mostly copying the ubitx or more simply a dual conversion radio?
NOTE ubitx design suffers from lack of filtering in many places and as a result suffers a lot of spurs
at varying points especially its outputs.
Using 5109 style amps work and well to with pads at their output as that helps stabilize the opposing part impedance. That is if the output is poorly terminated the input will be as a result and the most common source of that mistermination is filters (crystal or LC) at other than the working frequency.
the other thing to watch for is (TX mode) is correct gain distribution as the put out of the balance
modulator is fairly high and to much gain before the next mixer causes overload and bad things
happen. Since in a dual conversion scheme if you over drive the first mixer the second also
can have the same issue with unwanted products.
>>I've built up a Raduino, several of the classic SBL-1/2n5109 boards, and breadboarded things into nicely working 12 MHz IF output for the Hycas, but realize I don't fundamentally understand a few things I should about how I've tacked this all together so far<<<
The bigger issue is the SI5351 at higher loading has cross talk between outputs. The ideal interface
is one that invokes less current flow at the output. Lacking that a T type pad is somewhat better
with that part.
The goal is that diode mixers DBMs are sensitive to mismatch, but that vaires with which port.
he IF port being most sensitive and the LO least and the RF port is similar. That means a
SWR or return loss of more than 10DB is fine and pads 5db or less will satisfy that depending on
the load at the other side. For example a 6DB pad unterminated means the reflected power
from the unterminated port will go through and return to the source with a 12DB loss and
that represents a SWR of roughly 1.5:1 which is a satisfactory match to the source.
>>>Specifically I don't really understand the use of pads to terminate amplifiers and oscillators. <<<
Like transmitters and receivers both "like" a matched source or load and a terminating pad will
insure that at some know loss factor. In many cases the loss is not tolerated so we use matching systems
like antenna tuners for transmitters but for wide band systems that is not always a satisfactory way to proceed.
So we can use a Diplexer with one port terminated or IFs and other single frequency systems or a pad to insure termination quality is uniform for all frequencies.
>>>Routinely I see 6 db pads, but I'm not sure if there is magic in that, or whether a 3 db pad would be sufficient for the post amp, or if there is a known tradeoff. Specifically, in the case of the 2n5109 post mixers I'm using, I could pick up another 6 db of gain if I switched to 3 db pads in the two post amps, and trade that off for a better front end filter. Is it possible to explain qualitatively or quantitatively whether I'm sacrificing the impedance matching benefit if I drop to a 3 DB pad? The post amps look directly at SBL-1s for the PD and 2d mixer respectively.<<<
With pads the trade off is simple higher value better match... at the cost of lost signal. So you do a balancing act and often the best is usually 3 or 6DB where 3 gets us a OK match and 6 May incur excess loss. All very system dependent. Best suggestion is closer to the antenna (RX) likely the 3db is preferred, in a TX where
you can make up gain (smplifiers) 3 to 6DB works. NOTE: were loss is a concern alternative techniques
like diplexers are a good way to go.
>>Same thing with the SI series outputs. I sometimes see 6 db pads (or no pads) often used into level 7 mixers. In this case I'm measuring about 10 dbm from the Raduino oscillators. After duly reviewing EMRFD I see the mixer is probably pretty happy with 4 dbm if use a 6db pad, but at some incremental reduction in IMD, so if I just used a 3 db pad would that make the Si5351 sufficiently happy and meet the L7 spec?<<
SI570 and 5351 series sources are big enough to guarantee enough power to L7 mixers with usually up
to 5DB of pad. The mixer limits are about 4dbm to 13dbm especially in RX modes. For TX mixers higher power is usually better for lower IMD (in the 6 to 10dbm range).
Enough for one blast. I'm sure you will have more questions.
Allison