Introduction
I have looked at several designs and tried to collect some of the better and most effective solutions in this design. I see this as the “first” start. There will be improvements, I am sure. Feel free to use these ideas and if you have any suggestions please do not hesitate to send me an email, please put “FT225” as subject since this helps me to sort out spam!
I also want to point out that some instruments will be used. You can do a lot with your digital multimeter, but sometimes this is simply not enough!
When you read this, please also remember that my idea, and philosophy, is always to use a pre-amplifier mounted very close to the antenna trying to keep a decent S/N at the receiver. I know that many do not agree to this idea, and that’s up to them. It depends a lot about were you live and what kind of signals you have around you. Far away from cities there will almost never be any problems with strong signals but in cities it will not always work. In some countries the ham-bands are very crowded and this also set different specifications for the receiver system. It might be solved with different types of antennas and pre-amps with higher noise factor but also higher compression point!
The receiver system performance can in some cases depend on the pre-amplifier and if you do modifications you might not get the same results as I do.
This is a front-end for the FT225RD but these circuits could certainly also be used as a converter-stage in a stand-alone circuit.
The extension board
Before you can adjust any new PCB for the FT225 you should have an extension board. This is a passive PCB with the same type of PCB-connector as used in the FT225. In this way the new front-end PCB will be much more easy to adjust. I do not mean that one must make a extension board, but if you do not use a coaxial-cable to feed the front-end things will get complicated without an extensionboard.
You will find more details about the extension-board here.
The front-end test board
It’s probably not necessary to have a test-board but it will make it much more easier if any modifications or measurements should be done on the front-end PCB.
With the test-board it’s possible to run the front-end on the bench and not be forced to have the front-end-board mounted into the FT225. It’s possible to supply the front-end with an optional LO, RF and DC-signals.
You will find more details about the front-end-test board here.
Simulations and calculations
If you are really interested in RF-construction it’s a great advantage to be able to simulate circuits. I use a student version of the Serenade software available from Ansoft. There are many types of software and if you search the net you’ll find some different types and often the companies have some type of free version available for download. (The Ansoft student package is rather large, about 50Mb.)
Some of the constructions used in the front-end is simulated and zip-files are then available if you would like to perform similar simulations.
I have included several calculations on this page and the reason is that you can also use the front-end as a converter and if you do then it could be nice to recalculate component values for your IF, ie 28MHz etc…
- Mike Santori;
- “No one trusts the simulation except the engineer who ran the simulation. Everyone trusts the test except the engineer who ran the test...”
Front-end construction
Interface
As mentioned earlier the original feed from the antenna socket to the front-end is not used. It’s much more practical to have a thin coaxial-cable from the antenna socket and use a small coaxial connector as SMB or SMA. These connectors costs about 10USD per pair if not the highest quality is used, and most certainly worth this already the first time you need to do measurements on your board!
The cable can be of any type, the attenuation is not of any great importance if you have a pre-amplifier at the antenna. If possible use Teflon-cable since these are more easily to solder, double shield is preferred.
NB; Do not put any anti-paralleled diodes at the input, this type of diode-applications should be called “intermodulation creators”!
The mixer
When designing a front-end I think it’s best to first select the mixer. When the mixer is selected one can calculate how much power each amplifier-stage must be capable of handling and you will also be able to calculate the total noise-figure.
Most often the mixer is the most expensive component in this kind of construction and therefore you are also able to calculate the project costs, and as nearly always, quality costs!
There are several mixers to choose from. They are made by several manufacturers and I think that given a mixer-type with a given LO-power there will be no, or very small, difference in performance if you compare different manufacturers. Different types though will have different performance at the same drive-level or costs.
I decided to use a mixer from Mini-Circuits since they are relative easy to obtain. My choice is a "17S"-type, the
TUF-1H, capable of handling 14dBm on the RF-input, (under linear conditions). There are better types, and this solution is certainly not "uncrunchable", but the DC-power available on the 8V and 13,6V lines is restricted and therefore I decided to use only a 17S-type. The cost of this mixer is also about a tenth of what a 27dBm mixer costs! (“17S”-type means that this mixer will handle a slightly higher input-power than a ordinary 17dBm-type.)
With about 6dB loss inside the mixer, the output from the mixer will be about 8dBm as a maximum.
The 17S-type of mixer requires 17dBm of LO-power and it operates in the region of 2 to 600MHz. 144MHz will be in the "midrange" and therefore the isolation between LO-RF and LO-IF will be quite high, 30-50dB.
When we now know the mixer-type, we also know how much power the first stages may feed to the mixer, ie 14dBm. When we have selected input-stages it is easy to calculate when the receiver will be overloaded, ie how much power we can tolerate on the antenna input-connector.
If you want to be sure that you will be able to handle signals from a strong neighbour-ham operating on the same band you should make a link-budget.