BIGCAT RF Conversion and Scheduling
This guide describes how the BIGCAT RF conversion system works, from both the technical side, and from the user's perspective. The conversion system can be experimented with using the BIGCAT RF Calculator. While following along with this guide, it will probably be useful to visualise what is being described in the calculator.
The User's Perspective
To begin, we discuss the system from the user's perspective, and how most observers will interact with the conversion system, via the scheduler. At this point, the most important thing to remember is that there are four digitised bands, each 1920 MHz wide, and these four bands can be arranged in a limited number of ways with respect to each other. The calculator calls these the "Digitised Bands", because this is what is sent to the Jimbles, and thus what ends up being correlated and written to disk. From the user's perspective, the frequency coverage of these bands is all that matters.
Normally then, the user will want to know the frequency at the center of each of the four digitised bands. Some will want the four bands to be contiguous in frequency, whereas others may want them spread out for maximum sky frequency coverage, and still others may want specific subband frequencies for spectral-line work which may mean that overlap between the bands is required. The spacing of the four digitised bands is controlled by the LO offsets available in the BIGCAT RF system.
Each band (which are called Bands 1, 2, 3 and 4) has a bandpass filter that covers 2720 MHz of bandwidth. The Jimbles only digitise 1920 MHz of bandwidth (at least, that's all we make available to normal users). To select which part of the band is digitised, we have use the LO offset. These offsets can be set between 0 and 800 MHz, in 160 MHz steps. For example, for band 1 the bandpass filter makes available the native frequency range 4128 - 6848 MHz. With an LO offset of 0 MHz, the Jimble is sent the lowest part of this range, that being 4128 - 6048 MHz, which is 1920 MHz of bandwidth. With an LO offset of 160 MHz, the Jimble gets 4288 - 6208 MHz, and so on until with an LO offset of 800 MHz, the Jimble gets 4928 - 6848 MHz, which is the highest part of the bandpass filter range.
In practical terms then, for the 4cm receiver (which happens to be the native frequency range of the conversion system), each digitised band can only have 6 possible centre frequencies. These frequencies are listed in the table below.
| Band Number | Offset 0 | Offset 160 | Offset 320 | Offset 480 | Offset 640 | Offset 800 |
|---|---|---|---|---|---|---|
| Band 1 | 5088 | 5248 | 5408 | 5568 | 5728 | 5888 |
| Band 2 | 7008 | 7168 | 7328 | 7488 | 7648 | 7808 |
| Band 3 | 8608 | 8768 | 8928 | 9088 | 9248 | 9408 |
| Band 4 | 10528 | 10688 | 10848 | 11008 | 11168 | 11328 |
These are fixed numbers, and cannot be changed. Because this is the native frequency range of the conversion system, these numbers apply to all bands, but because we have tunable LOs for the 16cm and mm receivers, the sky frequency of these bands can be tuned as required. For example, at 15mm, we might want to have Band 1 tuned to have a centre frequency of 21720 MHz, using an LO offset of 480 MHz. To do this, we would tune the 15mm LO to a frequency of 27288 MHz. Because this is above the desired sky frequency, we have to digitise the lower sideband, and the frequency difference here is (27288 - 21720) = 5568 MHz, which is the number in the table above. This single LO applies to all the bands.
In summary, the user knows the centre frequencies of the four bands, and is happy, even if the choice is somewhat limited by the lack of LO at 4cm (and as we'll see later, a limited choice of allowable LO frequencies at 7mm). In the scheduler they can select their configuration and be confident that their selection will be enabled when they run their schedule.
The Scheduler's Perspective
The question then is, how should the scheduler communicate the user's choice to the observing machinery? This could be communicated by specifying eight numbers, those being the centre sky frequencies of each of the bands and the LO offsets. Let's complete our example above and work out the most efficient way of storing that information.
Our user has selected band LO offsets of 480, 480, 800 and 800 MHz (as we'll see later, this is what we call the "high" offset setting). The user wants centre frequencies of 21720, 19800, 17880, and 15960 MHz for each band respectively.
So the simplest way to communicate this is to specify these eight numbers: 21720, 19800, 17880, 15960, 480, 480, 800, 800. With the knowledge of the fixed table above, the observing machinery can look up that a band 1 offset of 480 MHz has a centre frequency of 5568 MHz, and thus because 15mm is lower sideband, the LO must be (21720 + 5568) = 27288 MHz. But you could also work that out from any of those frequency pairs, ie. band 3 gives (17880 + 9408) = 27288 MHz, so there is redundant information here.
The upshot of this is that a full specification of the frequency setup requires only a single sky frequency, and the list of offsets.
So which frequency should be chosen as the representative? You could specify the centre frequency of any of the bands. In the example above, you could have the representative frequency be 21720 MHz, that being the centre of band 1. However, technically this is two numbers, those being the frequency, and the band number.
Perhaps the most portable frequency specification then is the centre frequency of all the digitised bands in the configuration chosen by the user. This can be determined from just band 1 and band 4. In our example, the frequency of band 1 is 21720 MHz and the frequency of band 4 is 15960 MHz. The centre frequency of the digitised bands is thus (21720 + 15960) / 2 = 18840 MHz.
Working backwards now, if we are supplied with the centre frequency of 18840 MHz, and the list of offsets being 480, 480, 800 and 800 MHz for each of the bands, how do we work out the frequency configuration? Looking at the table above, we can see that the centre frequencies of bands 1 and 4 for those offsets are 5568 and 11328 MHz, making the centre frequency 8448 MHz. Because the frequency 18840 MHz can only be obtained in the lower sideband of 15mm, we can work out the required LO from these two numbers: (18840 + 8448) = 27288 MHz. Then we can work out the band frequencies from the table: band 1 = (27288 - 5568) = 21720 MHz, band 4 = (27288 - 11328) = 15960 MHz.
For this example then, the entire frequency configuration is unambiguously specified with the numbers 18840, 480, 480, 800, 800.
You might have noticed though that this requires another bit of assumed information, that being the sideband being used for that centre sky frequency. This only works if there is a single mapping between centre sky frequency and receiver sideband. Thankfully, for the BIGCAT system, that is true, and we'll go through the restrictions per receiver now.
The 4cm band
The 4cm band is very conveniently used as the native frequency range of the BIGCAT RF conversion system. Thus, there is no LO present for this receiver before the sky signal enters the conversion system. The only restrictions for this band in the scheduler is that the sky frequencies for each band can only be one of the entries in the table above.
Recommended contiguous continuum setting: 8128, 160, 160, 480, 480
Recommended max-coverage continuum setting: 8208, 0, 160, 640, 800
The 15mm band
After the 4cm band, the 15mm band is the simplest, because tuning is easy and after conversion the entire band can be seen in the lower sideband (because the LO frequency is higher than the highest band frequency).
The 15mm band can be considered to go from 15000 - 25000 MHz, but the scheduler needs to know the restrictions for the tunings for each band. To work those out, we go back to our native frequency range, where we see that the native centre frequency is 8208 MHz, and the native bandwidth is 9160 MHz. Thus, it is not generally useful to set a central sky frequency that is within half the bandwidth (4080 MHz) of the band edges. Put another way, the scheduler should not allow the centre sky frequency to be specified outside of the range 19080 - 20920 MHz, inclusive.
For the individual bands then, the allowed sky frequency ranges are:
| Band number | Low Sky Frequency | High Sky Frequency |
|---|---|---|
| Band 1 | 21400 (offset 800) | 24040 (offset 0) |
| Band 2 | 19480 (offset 800) | 22120 (offset 0) |
| Band 3 | 17880 (offset 800) | 20520 (offset 0) |
| Band 4 | 15960 (offset 800) | 18600 (offset 0) |
For different offsets, the suitable amount should be added or subtracted to the stated frequency, remembering that the band is lower sideband. So, if the user wanted an offset of 640 MHz for band 1, then the lowest frequency they could request would be (21400 + (800 - 640)) = 21560 MHz, and the highest would be (24040 + (0 - 640)) = 23400 MHz.
Recommended contiguous continuum setting: 19840, 160, 160, 480, 480
Recommended max-coverage continuum setting: 20080, 0, 160, 640, 800
The 16cm band
The 16cm band is an outlier, in that it can only use a single band, that being band 4, and only two offset settings in band 4 (0 and 160 MHz).
We consider the 16cm band to be useful between 1000 - 3300 MHz. That should allow then centre frequency tunings between 1960 - 2340 MHz. For offset 0, this is a restriction of 2120 - 2340 MHz, and for offset 160, 1960 - 2180 MHz.
When specifying a 16cm configuration, simply give the centre frequency of band 4, and its offset, with -1 for the bands not being used.
Recommended contiguous continuum setting: 2120, -1, -1, -1, 0
The 7mm band
The 7mm band covers 30000 - 50000 MHz, but the LO is tuned within this range, meaning part of it (the lower part) will be observed in the lower sideband, and the higher part will be observed in the upper sideband. The conversion system can only use a single sideband at a time. We need to work out the band coverage for each sideband.
With the LO set to as high as it will go, the maximum frequency observable in the lower sideband is 41472 MHz, so the lower sideband range is 30000 - 41472 MHz. With the LO set to as low as it will go, the minumum frequency observable in the upper sideband is 40428 MHz, so the upper sideband range is 40428 - 50000 MHz.
Converting these to central frequencies by adding/subtracting 4080 MHz, we can see the restrictions become 34080 - 37392 MHz for the lower sideband, and 44508 - 45920 MHz for the upper sideband.
For the individual bands then, the allowed sky frequency ranges for each sideband are:
| Band number | Low Sky Freq (LSB) | High Sky Freq (LSB) | Low Sky Freq (USB) | High Sky Freq (USB) |
|---|---|---|---|---|
| Band 1 | 36400 (offset 800) | 40512 (offset 0) | 41388 (offset 0) | 43598 (offset 800) |
| Band 2 | 34480 (offset 800) | 38592 (offset 0) | 43308 (offset 0) | 45518 (offset 800) |
| Band 3 | 32880 (offset 800) | 36992 (offset 0) | 45228 (offset 0) | 47118 (offset 800) |
| Band 4 | 30960 (offset 800) | 35072 (offset 0) | 47148 (offset 0) | 49038 (offset 800) |
There is an added complication to the allowed centre frequencies of each band for the 7mm receiver. Because the base LO is multiplied by 3 to get to the LO used for the first conversion, we can only use integer base LOs, otherwise we'd be trying to tell the LO to create a frequency of .333333 or .66666 recurring, which is not possible. So, for each offset, the specified frequency can only be an integer multiple of 3 away from the specified centre frequency. Because 160 MHz is not an integer multiple of 3, the check must be made taking into consideration the actual LO offset.
For example, in the table above we can tune above 36400 MHz for band 1 in the lower sideband, with an LO offset of 800 MHz. That means we could choose 36403 MHz with this offset, but not 36401 or 36402 MHz. And we could choose 36565 MHz with an LO offset of 800 MHz, because that would be 55 x 3 MHz higher than 36400 MHz.
However, if we instead used an LO offset of 640 MHz, the minimum frequency is 36560 MHz, which means we could tune to 36566 MHz, but not 36565 MHz like we did with an LO offset of 800 MHz.
Recommended contiguous continuum setting (LSB): 35726, 160, 160, 480, 480
Recommended max-coverage continuum setting (LSB): 35646, 0, 160, 640, 800
Recommended contiguous continuum setting (USB): 44428, 160, 160, 480, 480
Recommended max-coverage continuum setting (USB): 44508, 0, 160, 640, 800
Original: Jamie Stevens (12-Nov-2025)
Modified: Jamie Stevens (12-Nov-2025)