As part of my experimenting with building a double conversion up-converter I'm working on building a 10 MHz OCXO for the first LO. I'm using a 3.3vdc LDO voltage regulator to power the OCXO. This regulator is really simple to setup. And aside from the IC itself it just takes two small ceramic capacitors to be stable. When built the little DC-DC converter takes up less space than a US quarter. The downside of this device is HEAT. If input to the IC is > 10vdc it gets pretty hot trying to bring it output down to 3.3v. So I'm limiting the input to 10vdc. I'll probably eventually require this using an LM117 as those can take up to 40vdc input and run output as low as 1.25vdc (+-). The way I've used them in the past I added a potentiometer and some resistors and caps to create variable voltage output. But these devices can also just run a static voltage output by using a fixed resistor and some caps. The trouble with just about any of these is disapation of heat. The process of dropping 13.8vdc to 3.3vdc if not done properly can generate enough heat to burn your finger when touching the device or heatsink. So there's a balance that needs to be made. ANYWAY...Here's a photo of the 10 MHz OCXO with the LDO Regulator onboard.
Here's a shot of the LM117 variable supply I'm using to drop 13.8vdc to 10vdc which then drops to 3.3vdc that feeds the OCXO.
This is not the final configuration, this is just a test setup to verify the Variable Frequency adjustment of the OCXO using a voltage divider resistor network. See the adjustable 1k Ohm Pot for that.
The 100 MHz upconverter I've built seems to work VERY well. I don't have any complaints as it is right now. For the sake of experimenting, and trying to learn I've decided to try to increase the distance between the LO and the Pass Band. In the 100 MHz upconverter there's only 9 MHz between the LO and the final pass band. I want to try to increase that distance to have as much reduction of the LO in the final pass band as possible. (I don't think this is required to be honest, which is why I already mentioned this is for the sake of experimenting). I enjoy working with RF Filters so I thought this would be a chance to build two of my own (and spend some more money on an already completed project LOL). So the idea I'm going to try looks something like this block diagram below including the filter designs to reduce the LO(s) as much as possible. This is all subject to change, but I just wanted to post the idea I'm playing around with at this point. I will also have available a MK-3 by mini-circuits which is a frequency (doubler) Multiplier. I may play around with that, and then rework the filters again. But that's just something I was kicking around with. I could (I think) use it at the 19 MHz Mixer on the 10 MHz LO and end up with 29 MHz out of that mixer (I think), which would make it even easier to build the second LO BPF since there would then be 29 MHz from the 100 MHz second LO and the final pass band. That's just something I've been toying with in the back of my mind. Again I don't really thing any of this is needed at this point. My 100 MHz upconverter is working great (so far) even in the face of some very strong signals. I just want to learn more is the bottom line. Parts for this modification have already been ordered. So this is the next revision based on my 100 MHz up-converter noted previous on this blog.
SEE ALSO: 100 MHz upconverter, and Double Conversion upconverter
I have a need to build a steep skirted RF High Pass Filter. The filter I'm going to try to build isn't maybe the best/perfect option, but I'm looking at it more as a test and learning experience. So I can't claim thiis to be the perfect option.
With that in mind, and wanting to do this as cheaply as possible here are some of the details about this little mini-project. The FTDX-5000 has a fixed 9 MHz IF output. It's pretty difficult to obtain or build a HP filter with a sharp enough skirt where we don't have a high insertion loss, or not enough reduction in gain in the unwanted Lower portion of the filter when I'm using a 9 MHz input to the single-stage upconverter. In order to attempt push the boundaries a bit, I'm thinking I will try to use a frequency doubler/multiplier on the 9 MHz IF OUTPUT from the FTDX-5000. This will give me 18 MHz separation between the LO and the Pass Band, instead of only 9 MHz. This comes at a cost of -11db on the resulting 18 MHz signal from the original 9 MHz.
9MHz x2 = 18 MHz using MK-3 http://www.minicircuits.com/pdfs/MK-3.pdf $60.00 shipped.I don't think this will work! I'm pretty sure this requires a very high input level that the 9 MHz IF won't have.
It's probably better to think about doing a double up-conversion using a second OCXO like a 12.8 MHz = 9 MHz = 21.8 MHz instead! These 12.8 MHz OCXO's are 2x the cost of the 100 MHz OCXO I already have. But building the filters is much simpler since we have a larger band spread and things become a lot easier from a filter sharpness perspective.
The down side is that if we're not really careful with the filters we end up using we could actually be introducing MORE spurs, noise etc...
This also means I'll have to swap out the HF Band Pass filter I've *been using* (a Minicircuits ZX75-12+) - Probably another mini-project filter to do there too I guess. By the time this whole thing is done I'll probably had to build all of my own filters instead of getting off the shelf stuff. The only issue with doing that is that manufacturers and re-sellers these days are really pushing SMD/SMT parts. The last batch of parts I got were no larger than a spec of pepper. Literally, and I had to abandon that one and reorder larger inductor coils and ceramic disc caps.
The filter I'm wanting to build is intended to reduce the 100 MHz OCXO oscillator main carrier by 55-60 db below the 118 MHz+ Pass Band I want to make use of. Since I don't need this 100 MHz carrier in the pass band that flows into the Nooelec 0.5ppm RTL SDR Dongle I'm trying to eliminate it from being passed into the Dongle as much as possible. The idea being that I just don't want it to de-sense the receiver in the RTL Dongle.
Honestly I'll be pleasantly surprised if this has a noticeable positive impact on the 118 MHz+ Pass band desired.
The filter I've come up with reduces <= 104 MHz by at least -65db to -69db while only reducing 118 MHz+ by about -5.53db effectively 'insertion loss' at my desired frequency. So the net effect should be (-65 - -5.53) = -59.47db. That seems like a pretty good reduction of the LO showing up in the input to the RTL Dongle. Currently it's about +5db above the 109 MHz+ desired pass band where I'm not using a frequency doubler. The new HP filter I'm planning to build will have a -5.53db insertion loss at 118 MHz. That's the 100 MHz LO + the 18 MHz (9 MHz X2). For a total loss of -16.63db. I'm not too concerned about this at this point as I have an +22db LNA available for HF (LNA4HF).
(using Iowa Hills RF Filter Designer v2.2)
So what I'm hoping this might accomplish for the upconvertion process is to reduce spur images showing up in the Pass Band. (not that there are many at all, in fact at this point I can't really see much of that going on) So this is just a sort of research/test and development/learning process for me. I have an idea, and I want to test if it helps or not, or doesn't do anything all, or makes things worse. I've already added a 41db voltage variable attenuator. So there should be plenty of options for optimizing the RX chain. I'll post everything about this as begin this mini-project on THIS page. I've already ordered the parts and I'll post the parts list here asap with design details on the filter etc. This is a work in progress updates will be coming as I get them.
KEY POINTS
Iron Hills Filter Designer (Freeware) & Notes [Download]
