Filters For the 0 - 3 GHz Spectrum Analyser


Resolution filters (Final Crystal Filters)

Just over a year ago, many of us took advantage of the very generous offer to obtain the sets of crystals to build the resolution filters. Scotty followed this up with a note describing the nature of the crystals together with a warning concerning the fragility of the leads. These leads are very delicate and WILL break if you try to bend them more than once. Just very carefully straighten them and then solder them onto the board. Believe me, I broke two leads on my first attempt but managed to solder through the plated through holes to rescue the filter.

The excellent program Dishal 203 will guide you through the design stage but assumes that all crystals are the exactly same so be prepared to experiment with values. I used at least two caps for each value, one at about 75-80% of the target value and the other selected on test. To make it easier to change the caps, I solder two short pieces of wire (scraps from wire ended resistors) at each location and solder the C's to these until I get the best result. Then remove the wires and solder the SAME C's in their final place. You have in effect eleven C's and two L's to play with so be prepared to spend some time to get it right. The computer program will give you the filter impedance but do not assume that the circuits connected to the filter are 50 ohm, they will be close but will require tweaking to get the response correct. I found that the maximum bandwidth that Dishal designs for is about 7kHz so I am going to have to resort to commercial units to get the wider bandwidths. I haven't built the very narrow filter yet but I am aiming for about 500Hz. If anyone knows if greater bandwidths can be obtained using the sets supplied, please pass on how it can be done.

Good luck



If you just want a filter that can let you determine whether everything else is basically functional, you can build a 60 kHz bandwidth filter very simply, by cascading two ceramic filters and adding impedance matching components. The results are shown here:


That image used resistors to match, but you would want to use 1.8 uH inductors (Q of at least 20) leading to the filters and 100 pF shunt to ground at the filter terminals. You likely could get by without any fine tuning, though you would probably want to tune it later.

The wide filter limits the resolution of your scans, but there is very little chance that its shape will come out totally wacko, so it at least lets you see whether the rest of the MSA works. And when you have a full set of RBW filters, this can be one of them.

For narrower filters, a simple way to get started is to buy a canned filter on eBay, such as this one:

eBay filters don't always specify the terminating impedance, but this one does. The termination is specified as 2000 ohms,which you would match to 50 ohms with 4.7 uH inductors leading to the filter terminals and 47 pF capacitance shunt to ground from the terminals. To allow some tuning, use 33 pF capacitors and a small trimmer that goes to 25 pF or so. You don't need any equipment to tune it other than the MSA. Just install it as the RBW filter, and do a sweep with center=0 and span=0.02 MHz and you should get a nice graph of the top 50 dB or so of its response, which is all you need for tuning. Adjust the trimmers until the top of the response looks good.

For a PCB, you can use a blank double-sided board. Drill holes for the terminals and for a few ground vias. Mount the filter on the bottom and solder its case to ground. Put the components on the top with a fence running across the middle of the filter to isolate the input and output. Solder a fence around the outside of the board and solder a lid on it to cover the components. (If the fence is soldered to both the top and bottom ground planes, you can skip the ground vias described above.) It is not critical to solder the lid to the isolation fence in the middle of the filter. Poke/drill/cut two holes in the lid so you can tune the trimmers. You can cover the holes later with copper tape (or tack on a piece of brass), though it doesn't seem to cause any harm to leave them open.

I forgot the SMA connectors. Poke holes in the enclosing fence and solder the connectors to the fence with the center pin poking through the hole. A filter built with a similar approach is shown here:


Crystal Ladder Filters

I have not actually built a crystal ladder filter, but I have tested some crystals and used the results to simulate some filters. I'm looking at bandwidth in the 500-1000 Hz range. Before I started playing with the simulations, I assumed that getting a good response with a narrow bandwidth might require more crystals than for a wider bandwidth. Exactly the opposite seems to be the case, as the simulation results are very good for a 800 Hz filter with 4 crystals and a 500 Hz filter with 3 crystals. This is good news, as the smaller number of crystals obviously simplifies crystal matching and tuning of the assembled circuit.

Another interesting simulation experiment: I used 5 crystals and did away with all the series and shunt capacitors and put unity gain buffer amps between the stages. Four amps total--no amp at input and output. I made the source and load 50 ohms. Then I put a 100 ohm load at the output of each crystal. I got a very nice response with a bandwidth something like 900 Hz, and ultimate attenuation of at least 110 dB. I reduced all the crystal loads to 50 ohms, and the bandwidth narrowed to about 500 Hz. This general idea was discussed long ago, based on an HP design that varied the loads with PIN diodes in order to change the bandwidth. But it also is a nice simple design to get a narrow bandwidth which you can adjust during assembly to get your target bandwidth. For real-world crystals, the frequencies of the crystals can be adjusted to identical values with series capacitors, and there is no reason that the final response should have any ripple whatsoever.

It appears that the 500 Hz buffered filter could be done with 4 crystals and still get good response. This would require only 3 amps, which could be tiny inexpensive op amps, in order to achieve good power supply rejection. One stage could have gain to eliminate the filter loss.

Sam W.


Hi Sam,
As a side note, if planning to add gain within a filter, you could actually eliminate the need for the I.F. Amplifier in the MSA. Just make the total gain of the "Amplifilter" about 35 dB.

Crystal Ladder Filters

That would be a good idea if all my filters had this design, but I'm only going to do it for the narrowest. It could work for wider filters, but you would have to add more crystals.

I think I have found the optimal approach: Create two 2-crystal filters. Connect them with a buffer amp. This basically doubles the dB values of attenuation at every point, without creating any resonance interaction between the two filters, so each pair can be independently tuned. Connect 50-ohm source and load, and load the two inner crystals with 50 ohms. If you like what you get, adjust the amp gain to reduce the loss. If you want a broader bandwidth, then all the 50-ohms need to be increased (which will also reduce the loss). Because the amp is in the middle, it probably can only be used to offset the loss of the first half of the filter; otherwise the second half might get overloaded.

In simulations, it appears that if a crystal is mis-tuned, the loss increases and a small bump appears on the side of the response. Mis-tuning never seems to create ripple on the peak. Ripple only arises if the coupling capacitors between each pair of crystals are too small (i.e. too much coupling). So if you see ripple, you know exactly what capacitor to adjust and which way to adjust it.

Tuning a crystal is done with a series capacitor. Reducing the capacitance increases the crystal frequency. The idea is to get all crystals to the same frequency. If they are close to begin with, maybe no tuning is required.

The circuit does not seem very sensitive to input and output reactances. I put shunt 10 pF on the input and saw no change.

Sam W.