An antennaFirst thing you need is an antenna. For this purpose, I built a 14-element Yagi antenna, as seen below.
|14-element 1.4GHz Yagi antenna|
|Closeup of the amplifier next to the resonator of the antenna. The circuitry is hidden below the copper shielding.|
|S11 of the yagi antenna shown above. x is frequency, y is log reflected power (lower is better).|
A PreamplifierThe second thing in the signal path is a preamplifier, which is usually placed directly at the antenna and has the purpose of amplifying the received signal before any degradation can happen (e.g. through cables, which weaken the signal and add noise). The noise figure of the preamplifier (which is often called LNA, for low-noise amplifier) is important; it tells how much additional noise will be present in the detected signal. The noise figure of all subsequent amplifiers is usually not relevant: they amplify the noise added by the LNA, which is far worse than their own added noise. Thus, it makes sense to carefully design the very first amplifier, while the rest of them can be as cheap and simple as possible.
It is very difficult to design a good LNA though; so for some first tests, I use a generic wideband amplifier IC as my preamplifier, which has a not-very-good noise figure of 2dB. I hope it will work as a proof-of-concept; I plan to add a proper LNA later then.
More amplifiersThe next thing you want to do is to somehow amplify the received signal so much that its quality is limited by thermal noise: as soon as you reach that point, further amplification buys you nothing, you need to improve the antenna or cool your LNA. How much amplification that is exactly will depend on the circumstances, but since my measurement device can detect power levels down to roughly -120 dBm/Hz without problems and thermal noise from the antenna will probably be somewhere around -180 dBm/Hz, I will need somewhere around +60dB amplification at least -- better a bit more. That's about a factor one million more power.
Wide-band gain blocks are not a solution for every problem :(My first attempt to achieve this was to use several more wideband amplifier blocks -- since they're cheap (~3€) and incredibly easy to use (connect signal in, signal out, add 5V power, done). That wasn't a very good idea for two reasons though:
First, besides the noise figure, another not-so-obvious disadvantage of a wideband amplifier is that it is, well, wideband: it will amplify everything, especially for example the very bright GSM (mobile phone stuff) band around 937 MHz. That is bad, because such a strong signal can cause intermodulation products which affect the quality of the signal you actually want to detect, or even cause the amplifiers to go into saturation.
Second, it is very easy to turn broadband amplifiers into oscillators. You have to strictly separate them from each other and avoid any kind of feedback, or they will start spitting out large power levels at seemingly random frequencies.
Both problems seem easy to solve by building a bandpass filter -- but I found it quite hard to build a good bandpass filter for 1.42GHz. It is a frequency where lumped-element filters (those made out of capacitors and inductors) are not really viable any more since you need capacitors with incredibly small values, while distributed-element (microstrip, see this blog post) filters are not that great either because a half- or quarter wavelength of 21cm is still pretty large.
A new experiment: Frequency MixersAll those resaons taken together brought me away from using this simplest possible solution, so I decided to try what all the cool people do: use a frequency mixer to bring the signal down to a more manageable frequency after the preamplifier, then amplify and filter it at that low frequency. A frequency mixer is effectively a device which shifts the spectrum of a signal along the frequency axis: Mixing a 1GHz sine with a 995MHz sine will result in signals at the difference (and the sum) of those frequencies, so a 5MHz sine (and a 1995MHz sine, but that can easily be filtered away -- if you don't want it). Signal components which are shifted below 0Hz appear mirrored at the appropriate positive frequency (so, if a signal would be shifted to -3MHz, it will appear at 3MHz instead -- with a phase shift, but that is barely relevant).
Mixers are very versatile devices which are used basically everywhere where radio frequency signals are present -- for example in WLAN, in GSM, radio, or satellite television (in the latter, there's actually a mixer in the LNA thing which you put in front of your satellite dish which shifts the received ~10GHz signal to somewhere around 2GHz to reduce losses in the cable to your TV receiver).
Professional radio astronomy applications tend to use more than one mixer stage to lower the frequency gradually (that has various advantages mostly related to filtering), but I hope I can get away with just one stage for this project.
I thus built a test board based on the LT5560 mixer (~3€) which is designed to mix a 1420 MHz signal down to 20 MHz using a 1400 MHz local oscillator (LO, that's the name for the frequency you're mixing the signal you're interested in with). As the LO, I use a programmable frequency synthesizer I built a while ago which is based on the ADF4350 (~8€, but wow that thing is difficult to put on a working board -- took me three attempts with a new board design each).
|ADF4350-based frequency synthesizer (left) controlled by a raspberry pi (right). This construction is used as the Local Oscillator (LO) for the mixer. A 1399.5 MHz signal is produced by this circuit on the coaxial cable on the left.|
Balanced and Unbalanced signalsApart from input matching, which is not very difficult to do since it is described in detail in the data sheet of the part, the one difficult thing which needs to be done when using a mixer of this type is that it requires balanced inputs, while the signal on your coax cable is usually unbalanced. The difference between those two is basically that the unbalanced signal has two voltage levels -- ground and the signal, while balanced has three, plus the signal, minus the signal, and ground. A device which converts one of those into the other is called a balun (for balanced-unbalanced). They always work both ways (balanced - unbalanced or unbalanced - balanced). A balun is usually made in one of two ways:
- transformer type: the unbalanced signal is passed into a transformer and by clever choice of taps on the load side you can get a balanced signal
- delay line type: transmission lines of certain fractions of the wavelength of the expected signals (e.g. 1/4 and 3/4) are connected to the balanaced signal, and tap different phases of the wave form because of their different length, which allows to extract a balanced signal
I will not go into details on how to build those baluns here, but good documents describing how to do it include this, this (german), this and this (ready-made components are available and are not even very expensive, but I did not find any Europe-based distributor who actually sells them. And I don't want to pay $20 shipping fees and wait a week each time I need a part).
I checked the performance of the self-made baluns with a spectrum analyzer and directional coupler, and they seem to work well after a few experiments -- although I am not entirely happy with the performance of the transformer-type one: a return loss of -13dB means about 1/20 of the signal power will get reflected back into the mixer, which is more than I would like. I think one reason for this is that the windings of the wire are a bit chaotic which makes the coupling between the windings a bit non-deterministic. Still -- all good enough for the prototype ;)
The S11 curve of the delay-line one is much sharper than the one shown here, and it also has a better return loss at the design frequency.
|S11 (return loss) of the transformer-type balun; x axis is frequency, y axis is log power|
|S11 of the delay line type balun. The resonance frequency around 1.4 GHz is easy to spot. The ripple at lower frequencies is probably at least partly an artifact of the measurement method. x axis is frequency, y axis is log power|
But most important, it actually works:
|LT5560 active mixer test board. Yes, it looks quite horrible because lots of repairs were needed after the initial fabrication ;) Next one will look nicer.|
|Mixer output for 1.3995GHz LO and 1.405 GHz input. If the input frequency is lowered to and then below the LO frequency, the frequency peak moves to the left until it hits zero frequency and then starts moving right again.|