Sunday, April 29, 2007
Testing ERA-5 MMIC for use as a multiplier
ERA-5 gain below 1GHZ = 20 DB
1 DB compression = +17.5 DBm
Input 738 MHz into MMIC at +6 DBm and +2 DBm drives the amp well into compression. No attempt was made to optimize the output for a particular harmonic or to suppress the fundamental. The output capacitor was 12 pf to favor the upper frequencies. Results varied depending on the input frequency. The data below is for 738 MHz. At 571 MHz the 5th harmonic with +2 DBm input was -20 but had a lot of “grass” around the carrier. Lowering the input to +1 DBm dropped the harmonic to -30 DBm and cleaned up the noise (or it dropped below the noise floor of the spectrum analyzer).
It appears the input should not exceed 5 DB above the 1 DB compression level or excessive noise will result.
+6 DBm input is 8.5 DB above the 1DB compression point
+2 DBm input is 4.5 DB above the 1DB compression point
Output 1 is with a +6 DBm input
Output 2 is with a +2 DBm input
Frequency(MHz) or Harmonic/Output1/Output2
738/+20/+18
X2/-2/-6
X3/+3/-12
X4/-4/-30
X5/-20/-30
X6/-42/-34
X7/-38/-46
X8/-30/-42
X9/-40/-50
X10/(below noise)/(below noise)
Tuesday, April 24, 2007
Oscillators
Other changes I made:
1. Removed the tuned transformer between the oscillator and buffer. Replaced it with a short to the DC line.
2. I moved the input capacitor of the buffer that was on the transformer link to the top of the oscillator emitter resistor.
3. I also reduced the buffer emitter resistor from 220 to 100 ohms. This helped reduce the harmonics a little. I think the buffer was also being overdriven.
I experimented with the tapped capacitors in the oscillator tank circuit. Increased them to 82 and 33 from 68 and 22 pf. No real difference. The oscillator may be slightly more stable, from a start up perspective, with the original caps so I put them back to the way they were.
In the end the 2nd harmonic is 10 db down and the others are 30 db down or more. The 5th harmonic is the last one I see on the spectrum analyzer. The input tuning on the multiplier may make these spurs a moot point. If not I will have to add an extra low pass filter after the oscillator. The final oscillator is much simpler design.
I will incorporate the design changes into the 114.2 MHz oscillator and test it out tomorrow night.
I guess I will have to go back and update the design document on my web page. That will have to wait until after Hamvention. I need to get this project done by Hamvention.
Interdigital Filters - remade
Friday, April 13, 2007
Making New Artwork for 738 Interdigital Filter
Update on Frequency Error in Ansoft Design
Measurements on the N4 Interdigital Filter
P (total length of each microstrip, there are four) = 2062.5 mils
P2 (tap for the input and output from ground end) = 203 mils
W (width of each microstrip, they are all the same) = 164 mils
S1 (spacing between element 1 and 2) = 391 mils
S2 (spacing between element 2 and 3) = 172 mils
S3 (spacing between element 3 and 4) = 500 mils
Measuring the actual center of the band pass on the tracking generator indicates 737.68 MHz. Now we can modify the simulation with our actual measured values and keep changing the dielectric constant until the frequency matches the measured. Nominally FR4 is around 4.5. The simulation shows:
For 4.7 Fo = 743.53 MHz
For 4.8 Fo = 736.39 MHz
For 4.9 Fo = 728.57 MHz
For 4.79 Fo = 737.68 MHz
Setting the simulation to 4.79 yields our measured frequency of 737.68. We will use this value for future filters around this frequency. I want to try a hairpin filter next. I have the initial design. Other than the narrow spacing and I assume more critical layout tolerances the pass band does not contain the “step”. Hairpins do respond to the even harmonics of the first pass band. I don’t think that will be a problem the higher harmonics of the multiplier will be reduced by the design of the multiplier.
Saturday, April 07, 2007
Documenting the shortening of the MicroStrips
Start length = 61mm
Pass band frequency per length of each microstrip:
61mm = 630 MHz
60mm = 642.5 MHz
59mm = 657 MHz
57mm = 678 MHz
54mm = 715 MHz
53mm = 728 MHz
52mm = 736.7 MHz
After Microstrip Adjustments
I have made the modifications to the 738 filter PC board layout. I shortened the strips to bring the center frequency to 738 MHz. The original length from the Ansoft calculations was 61mm. I had to shorten these to 52mm. That is a reduction of 14.75%. My guess the reduction is required because of stray capacitance on the board. The final 3DB bandwidth comes out to 15.1 MHz. The final gain with the +20 DB MMIC is 0DB. My insertion loss is 20DB behind that amp. This is in line with Ansoft’s calculations using FR4 board. Rogers PC board would be a better choice because of the dielectric loss tangent (TAND). It is 0.02 on FR4 but only 0.0009 for Duroid 5880.
The input return loss is interesting. Using FR4’s TAND value Ansoft shows the best return loss as only 10DB but above Fo at 756 MHz. This is less than the original 18DB I had before mucking with the strips. I see this on my tracking generator. I was able to improve this to 15DB by adjusting the tap point on the first microstrip element. I moved it a millimeter or two toward ground. I don’t know that it improved return loss at 738 as that still looks to be 10DB.
738 MHz Bandpass Microstrip tuning
I was playing around with Ansoft this morning and the 738 filter. First I adjusted the dielectric constant to see if I could account for the lower filter frequency response. I found the dielectric constant would have to be something like 7+ to drop the frequency down 100 MHz. Design spec for FR4 at lower frequencies is 4.5. I don’t think 7 is a reasonable value even at 700 MHz. I then checked my actual dimensions of the micro strip. They are within 0.5 millimeter of the design values. Again, not enough to account for 100 MHz. I am beginning to wonder if stray capacitance may be lowering the frequency.
Another finding was on the insertion loss. When I modeled this board I did not set the dielectric loss tangent value. I see from documentation that this should be 0.02 for FR4 board. Doing that I can now account for the loss. The insertion loss is 18 dog biscuits (Jake is pawing me as he wants some of those). That agrees with my board. My MMIC amp on the output has a +20 gain at 700 MHz. That gives me the +2 DB output with a 0-DBm input signal I see on the analyzer. Using
Next I will shorten the strips to see if I can bring the frequency up to 738. I can try to use the same technique I used for the pipe filters to tune each stage by lightly coupling a spectrum analyzer to the first stage then short out the other stages as I tune down the line.
Sunday, April 01, 2007
First Microstrip Bandpass Filter Results
I etched the board for the microstrip band pass filter. The desired center frequency is 738 MHz. The problem with designing such a filter on FR4 PC board stock is the variation of the dielectric constant at the desired frequency. The second source of errors in the layout of the board itself. At UHF and above it is difficult to get the necessary precision where a quarter of a millimeter could make a big difference. My results of the first filter look promising. The filter response was about what I wanted except the frequency was off by 100 MHz. The center of the bandpass was 630 MHz and not 738. I think the insertion loss may be high but it is masked by the output MMIC amp I have on the board. The overall gain is +2 DB because of the amplifier. While this does not sound bad remember that the MSA-0886 amp has about 20 DB of gain at 600 MHz.
The plot of the filter response shows a “step” on the upper frequency side of the curve. This indicates one of the stages is not aligned on the same frequency as the others.
Anyway, the initial measurements:
Input return loss at center of bandpass: 18 DB
Overall gain with MMIC amp: +2 DB
Center frequency of band pass: 630.68 MHz
3db bandwidth: 638.33 – 623.64 MHz = 14.7 MHz
The filter was designed using Ansoft Designer SV (student version). I will now go back to the design and modify the FR4 dielectric constant to see if I can account for the 100 MHz frequency error. The idea is to use the modified constant to rework the board dimensions to see if I can get closer to 738 MHz and improve the insertion loss.
As for this board it has served it purpose. I can try to shorten the microstrips and use capacitors to tune the sections. I can not adjust the coupling other than to solder metal tabs onto the microstrips and use them to increase coupling to the next strip.
Saturday, March 17, 2007
Tinkering With That Darn Oscillator
It appears the high harmonic output is generated by the last MMIC amp. Not sure why unless it is sensitive to the impedance seen by the input looking back into the filter. There is a simple low pass filter after the oscillator buffer. That filter should have been after the MMIC amp but the amp was an after thought when I designed the PC board. The harmonics go away when I remove the last output capacitor from the filter. I then have the second harmonic down 35 DB. I attempted to replace the first and last capacitor with trimmers to see if I could improve the situation. Not really, so I left it as it was minus the last capacitor.
I adjusted the oscillator transistor bias resistors to improve the output while maintaining the same 35 db harmonic attenuation. The oscillator output level is now zero DBm.
That still puzzles me as to why I am getting the harmonics. The MMIC is not being overdriven. The MMIC is an Agilent ABA-53563 I got from Mouser Electronics. The 1-DB compression point is +14 DBm output. I appear to have around -15 DBm going into the MMIC so with a 21.5 DB gain that is below the 1-DB compression point. I checked my spectrum analyzer calibration against my signal generator. The spectrum analyzer reading agree with the settings of the generator.
I will leave the oscillator as it is and get back to testing the frequency calibration and oscillator heater. Something tells me I will be redesigning the PC board moving the filter after the MMIC.
Thursday, March 15, 2007
Oscillator Oven
I have been working on a few oscillators for a radio project. The test oscillator is crystal controlled on a carrier frequency of 104.727270 MHz. This will be multiplied up to a higher frequency. Unfortunately I forgot what oven temperature I ordered the crystal for. I think it was around 50C. I did not want it to be too high as I am using Styrofoam 3/4” thick for the insulating material. The entire oscillator is heated. The oscillator is a
I now have a stable operation at a sensor reading of 3.292 V which equates to around 56C. The frequency counter reads 104.727284 MHz. The counter is locked to 10-Mhz GPS disciplined oscillator to ensure a correct readout. I am 14-hz off frequency at this point. The crystal changes about 57-Hz for a 2 degree C temperature change. This is around 0.54 PPM. The oven heater resting current is 0.06 Amps. The heater is nothing more than a power FET attached to the side of the aluminum oscillator housing. A thermistor on the oscillator PC board in the gate circuit controls the FET. An external pot in series with the thermistor is used to adjust the temperature.
Now I will remember to write down the temperature specification when I order any new crystals. 55 degrees C. works out well for the type of heater and insulating material I am using.
The next test will be to see how well the heater can hold this temperature when I subject the oscillator package to a temperature change. I need measure the external and internal temperature while the environment changes. I can do this by using the GPIB interface on my DVM for the on-board sensor and the RS-232 interface on my Radio Shack hand held DVM. The sensor is a LM324. I hope to be able to hold +/- 50 hz from room temperature to an ice bath. That would be 2 degrees C temperature variation.