[hpsdr] HPSDR Preselectors and Front Ends

[Steve Hubbard]

Hi Peter and Phil,

As this is an area where I've had some design experience, I look forward to 
seeing the preselector notes on the wiki and hope I can make a useful 
contribution. Most of my experience has been with pre/postselectors for 
marine applications where simultaneous transmission and reception has been 
required. Obviously the scope for separating the transmit and receive 
antennas is limited on a ship! I hope the following is useful.

The units I worked with had to provide at least 30 dB of attenuation at 5 % 
frequency separation with an IP3 of at least 55 dBm (5/10 %). This was 
achieved with two pairs of coupled resonators with an amplifier in between 
to compensate for the losses. Tuning was by means of reed-relay switched 
capacitors for speed and automation (note that in standard reed relays the 
RF resistance is too high for this application). Ganged variable caps or 
fixed tuning would perhaps be more appropriate for an amateur application.

The coils were initially solenoidal (pie-wound for the low bands) with iron 
powder screw cores for adjustment but good performance was later also 
obtained with the L57 'cup and screw core' formers from Lodestone Pacific. 
These latter formers have the advantage that they're are screened. The cup 
and core are made of iron powder, either carbonyl E for low frequencies, or 
SF for high frequencies. Iron powder was found to give much better 
intermodulation performance than ferrite and it did not drift much over 
temperature. In order to get the best Q at 1.5 - 3 MHz stranded silk covered 
wire had to be used. An enormous number of strands seemed to be needed for 
best performance. Litz wire would probably be best but I don't think it's 
easy to obtain these days. I never really tried iron powder toroids due to 
the difficulty in tuning them on a production line but I imagine they would 
give good performance and can be tuned by bunching up or spreading out the 
turns. The use of high Q solenoidal coils demands excellent overall 
screening and filtering in the unit to keep out switch mode noise etc. In 
this respect they appear to make excellent receive antennas! I had to fit 
two of these units in the same drawer as a switch mode PSU but reckon the 
effort took several years off my life!

To maintain a good filter response and insertion loss, both loading and 
coupling of the resonators have to be maintained across the band. Not too 
difficult over 1.5 octaves or so.

If transformers are required ahead of the filters, high permeability 
ferrites introduce less IMD. I found that the monolithic ceramic plate 
capacitors of the NP0 and N150 etc grades introduced unacceptable IMD but 
never worked out why. Multi layer COG ceramics appeared to be the best 
ceramic types; better even than silvered mica in this respect. Keep in mind 
however that I was looking for a level of IMD performance that most land 
based systems would never need. These effects are relatively minor.

When it comes to measurement of the intercept point of a preselector, I 
agree that it is challenging. The dynamic ranges involved stretch the 
capabilites of a spectrum analyser to the limits. Even if a hybrid combiner 
with isolated input ports appears to give good results on a spectrum 
analyser, it may produce worse than expected results when connected to a 
preselector. The reason for this is that a spectrum analyser has a broadband 
matched input but the preselector will totally reflect the out-of-band test 
signals back into both signal generators where they will often happily 
intermodulate. The best way around this I found was to start with as good a 
pair of signal generators and hybrid as possible and then to place a sharp 
notch filter before the unit under test. It is possible to make notch 
filters that can be tweaked to provide perhaps 50 dB or more of rejection at 
the notch frequency, thus rejecting the intermod product generated by the 
signal generators and hybrid, whilst presenting a 50 ohm termination at both 
ports. These filters were a bit tricky to set up and would go off tune if 
breathed on but just needed to remain stable for the duration of the 
measurement. The filters used comprised a parallel RLC circuit in series 
with the signal path with a series RLC to ground from a centre tap on the 
parallel circuit. It's a sort of bridge that has theoretically infinite 
attenuation at the notch frequency if everything is balanced.

Just a final point. Spare a thought for the humble attenuator. For transmit 
you need as efficient an antenna as possible but for receive it only needs 
to bring in enough signal to bring the external noise (atmospheric, 
man-made, galactic etc.) to just above the receiver's noise floor. Much more 
than this will exacerbate intermodulation problems.

Phil's right: Western Australia in the depths of the sunspot cycle is the 
last place you need a preselector.

Enough for one post.

73s

Steve VK6ABZ 


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