So, just what is ‘an intermod’ then?
by Pyers Easton
Anyone who has ever used wireless audio will have heard of ‘intermod free plans’, but many users do not know what this means. In this article I will attempt to explain.
Intermodulation (or ‘intermod’ for short) is a phenomenon that affects wireless where there are two or more signals present. The term ‘intermodulation’ literally means one or more signals modulating one or more other signals. This undesirable effect produces a number of extra, unwanted signals.
Intermodulation is most definitely not a wireless audio user’s friend as it sterilises channels that could otherwise be used to cram more channels into a given location.
If you look at Fig 1 you will see an example relevant to PMSE wireless users. You will see that there are two wireless signals, one on 609 and one on 610MHz, either side of these two signals you can see two extra signals at 608 and 611MHz, these are the intermodulation products (in this case third order intermodulation products, I have not shown 5th and 7th order products as they tend to be much lower in level). Third order intermod products occur either side of the wanted signals, spaced the same distance away as the signals are apart, in this case the signals are 1MHz apart, so the unwanted signals are 1MHz above and below. Obviously the more signals we have, the more intermodulation products occur.
If we look at Fig 1 again, it is probably clear that it is not a very good idea to place a wireless signal every 1MHz as the intermod products would fall on the wanted signals. Trying to use a channel with an intermod product on it will result in reduced range and a noisy signal. This is why we use ‘intermod free plans’. These plans are carefully calculated to ensure that no intermod between any of the channels falls either on, or within the receiver bandwidth of, any of the channels. This is why the spacing between channels in these plans is never uniform.
Let’s consider how and where intermodulation occurs: in free space two signals will exist happily together, won’t interfere with each other and produce no unwanted extra (spurious) signals. In the real world intermods occur whenever the signals pass through, or enter, a non-linear device, such as any practical amplifier stage. There are three main reasons for intermods:
- In a receiver: All receivers will have amplifiers at some point after the antenna input (such as are used for making up for loss in antenna cables). It is when these amplifiers are non-linear (normally due to poor design) that intermods between all the signals entering the unit will be produced. Most receivers have filters tuned to the desired receiving frequency at the input that reduce the bandwidth entering the amplifier; the narrower the filter, the fewer frequencies that can enter, and therefore fewer intermods are produced. All receivers will have a maximum signal level above which the active circuitry will become non-linear and therefore produce serious intermods due to overload.
- Tips to minimise this: firstly make sure you are using a top quality receiver, preferably with ‘tracking’ filters on the input. If you must use an antenna head amplifier, make sure it’s a very good one, with a very high maximum signal handling limit. To avoid receiver overload try to avoid transmitters coming within a few feet of the receiver, especially when you are trying to receive talent that is further away. If someone walks up to you during a shot with their TX on get them to turn it off! Also bear in mind that intermods can happen in other peoples’ kit, just because you have the best kit doesn’t mean you are immune to the effects of other kit nearby, so if someone is on set is using a £50 radiomic from eBay either move well away or drive a lorry over it. I recommend the latter.
- In a transmitter: In addition to the wanted signal that the transmitter is producing and sending out through the antenna other signals from nearby transmitters enter the device through the antenna and intermodulate with the wanted signal, normally in the output amplifier semiconductor. Some transmitters have a device called an ‘isolator’ (AKA ‘circulator’). This is basically a one-way valve, allowing signals to exit but not enter the transmitter, though these tend to limit the tuning range to around 30MHz maximum due to their narrow band operating range. Intermods between transmitters are particularly noticeable when several wireless transmitters are operating physically close to each other.
- Tips to minimise this: keep the transmitters on your talent as far apart as possible, i.e. if you have two presenters standing next to each other put the transmitters in pockets on opposite sides of their bodies. Only use transmitters with circulators or other intermodulation limiting circuitry if possible. If there are other people using wireless on set make sure that their kit is good too, and that they follow the advice above or their intermods could affect your reception!
- The ‘rusty bolt’ effect: Although intermods generally only occur in active circuitry, they can also occur in other ways, such as loose connectors or at junctions between rusty metal, this effect is known as ‘passive intermodulation’ (PIM). The level of intermods produced passively is generally less significant.
- Tips to minimise this: Use good quality antennas, cable and connectors. Make sure connectors and cable joints are clean and free of corrosion. Try to avoid using wireless on the Blackpool Tower (!).
In conclusion, intermods are a royal pain. As we are squeezed more and more by other users of RF spectrum the possibility of intermods prevent us using as many channels in a given frequency band as we could if they did not exist.
As is so often the case in life, you get what you pay for. I have always repeated the mantra that ‘intermods only happen in poorly designed equipment’, so when choosing your equipment please think long and hard before saving a few pounds in the short term on buying kit that may constrain your ability to work efficiently for many years ahead.
Some manufacturers are coming up with some very clever ways to eliminate the spectre of intermods, leading to the holy grail of just turning up at a site and putting up to 38 channels in an 8MHz TV channel, equally spaced. Many digital manufacturers offer high density operation, but this is usually at the expense of operating range and audio quality due to compression. Currently Wisycom are offering products which actually increase range with little or no loss of audio quality, their new LinEar TXs have virtually eliminated reverse intermods, whilst still tuning over nearly 200MHz, and their receivers offer narrow band operation which allows a radiomic channel striped every 200kHz. Contact Raycom for more information about this radical new technology.
Pyers Easton is Managing Director of Raycom Ltd and has been working in RF and audio design for over 30 years.