The Signal Designer

The signal a MOD_Source plays - LTE, NR, CW, a pulsed-CW train, FSK, Bluetooth LE, LoRa, Sigfox or your own I/Q file - with its spectrum, CCDF and EVM before you simulate.

Opening the Designer

The Signal Designer edits the signal a MOD_Source plays in an Envelope analysis. Select the source and press Design signal… in the inspector. The left of the window defines the signal; the right previews it. Every edit previews by itself after a short pause.

Generation runs in the background — shown in the window's footer and on a live row of the Messages pane — and is cached. A simulation of the same signal reuses the cached waveform instead of generating it again. Apply writes the signal to the source as one undoable step; Apply & Close does the same and closes the window. At the top of the left pane, Export signal saves the signal as an I/Q file and ? opens this page.

Tools ▸ Signal Designer opens the Designer on its own, with no source behind it and no workspace needed. It starts on one LTE 20 MHz carrier. Design a signal there, or read one in with Load custom to see its spectrum and CCDF, and Export signal to save it as an I/Q file. There is nothing to apply it to, so Apply and Apply & Close are not shown, and a file you load is remembered by its full path. Several can be open at once, and each is listed in the Window menu.

The Source box

• Centre Fc and Pavg (dBm) — the centre frequency the carriers sit around, and the total average available power behind the source impedance. Both accept a number or an expression (RFfreq, Pin), which the Designer writes back exactly as typed, so a swept source stays swept. For a pulsed-CW source the field reads Pulse-top power (dBm): the power on the flat top of the pulse, with the record average and the power duty shown beneath it.
• Record (ms) — the periodic record length, in whole 0.5 ms slots. ACPR settles within about 0.1 dB from 0.5 ms; a longer record tightens the statistics.
• Sample rate — the rate of the preview and of the export. Auto picks the recommended rate for the signal (122.88 MS/s for LTE 20 MHz); the list holds the standard rates from 7.68 to 1966.08 MS/s, and, while a custom I/Q file is loaded, File rate — the file's own. The line under it shows the rate in use, the sample period and the samples per record. A rate that cannot hold the signal is refused, naming the lowest one that can. It is a Designer setting: it is not written to the source, and the simulation picks its own time step. It is not the file sample rate of a custom I/Q file, which says how the file was recorded.

Presets

One click replaces the carriers with a common case. The presets are in one row per family:

• Cellular — LTE 20 MHz, NR 20 MHz (30 kHz), NR 100 MHz (30 kHz), 4 x LTE 20 contiguous, or 2C NR 20 at spacing: two NR 20 MHz carriers the number of MHz apart in the field beside it, centre to centre.
• Radar — Pulsed CW, one pulse train.
• IoT — each sets one carrier and moves Centre Fc to the mode's channel; if Fc is an expression, it is left as you typed it.
  • Wireless M-Bus — a menu of the FSK metering modes.
  • Bluetooth LE — a menu: LE 1M / LE Coded and LE 2M, as GFSK at 2.44 GHz.
  • LoRaWAN — a menu: a LoRa packet at SF7 or SF12, 125 kHz at 868.1 MHz (EU868).
  • Sigfox — the RC1 uplink: DBPSK at 100 b/s, 868.13 MHz.

A custom I/Q file

The Custom I/Q card makes an I/Q file you supply the whole signal: a capture from an instrument, a waveform you wrote yourself, or one you exported here earlier. Press the file button at the left of the card and pick the file. It is played around Fc at the power Pavg sets, exactly as a generated signal would be. While no file is loaded the card is empty and the carriers make the signal.

The file. Plain text, one sample per line: the in-phase and quadrature values, separated by spaces, a tab or a comma. Lines starting with # or % are comments, and two of them are read:

• # sample_rate_hz 200e6 — the rate the samples were taken at;
• # channel_bw_hz 40e6 — the channel bandwidth. Without it, the bandwidth is measured from the file: twice the furthest edge of the band holding 99 % of the power, over 0.9.

The scale does not matter: the samples are normalised to unit mean power and Pavg sets the level. The file is one periodic record, so its length is the record: 200,000 samples at 200 MS/s play a 1 ms record, and the last sample wraps to the first.

The Custom I/Q card shows the file, its sample rate, and what was read: the samples, the record and the channel bandwidth.

• File sample rate (MHz) is the rate the file's samples were recorded at. It is filled from the file's # sample_rate_hz line when it has one. A file with no such line cannot be played until you enter its rate here — the footer says so. A rate you enter wins over the header. Type a number in MHz (122.88) or a number with its own unit (1.5 GHz, 30720000 Hz). An entry that is not a rate is outlined, its tooltip says why, and the signal keeps the last valid rate.
• The × at the right of the file removes it. The signal goes back to the carriers it had before you loaded the file (one LTE 20 MHz carrier if the source was opened on a file), and everything the file disabled is enabled again. Apply then takes the file's reference out of the schematic.

What is disabled, and why. While a custom file is loaded, the Presets, the Carriers and Record are greyed: the file is the whole signal and its length is the record. Spectral shaping is greyed too, as it shapes generated OFDM symbols. Everything else still applies:

• Clip to PAPR clips the file to a target peak-to-average ratio, by the same in-band peak cancellation a generated carrier gets;
• Centre Fc, Pavg and the preview's Sample rate;
• the spectrum, the CCDF and Export signal.

Two sample rates, two jobs. The file sample rate describes the file and is saved with the source. The Source box's Sample rate is the rate the Designer previews and exports at, and is not saved. A custom file is resampled to the Source rate for the preview and the export — band-limited and exact, since the record is periodic — so a file recorded at 200 MS/s exported at 245.76 MS/s holds the same 1 ms record in 245,760 samples. To preview and export the file at its own rate, with no resampling at all, choose File rate in the Source box's list: it is offered, second after Auto, for as long as a custom file is loaded, and follows the file sample rate when you change it. A Source rate that does not give a whole number of samples over the file's record is refused.

The EVM tab has nothing to show for a custom file: it carries no reference symbols to compare against.

Where the reference is kept. The schematic stores the file the way it stores a Touchstone file: relative to the workspace when the file is inside it, or at most two folders above it, and as a full path otherwise, so a workspace you move or share keeps finding it. In the netlist the source's carrier reads:

Carrier[1]="FILE;File=iq/sc_64qam_40Msym.txt;Fs=200MHz;Off=0;P=0;Seed=1"

Editing the file. A file changed on disk is read again on the next preview or run; the cache never serves the old one.

In the Envelope analysis the record is stepped at the usual LTE / NR rate (1.92 MHz × 2ᵏ) when that divides it. When it does not — 12,345 samples at 100 MS/s, say — the step is the file's own rate × 2ᵏ, which always gives a whole number of samples per record.

Clipping and spectral shaping

• Clip to PAPR (dB) — off, the signal keeps its natural peak-to-average ratio (about 11–12 dB for OFDM). On, iterative in-band peak cancellation brings the peak down to the target, measured at 8× oversampling so peaks between samples count too. Clipping adds in-band error, which the EVM tab shows.
• Spectral shaping floor (dBc) — a ceiling outside the occupied band, so the source's own ACLR sits far below the amplifier's and what the simulation reports is the amplifier.

Carriers

Each card is one carrier: its standard (LTE, NR, CW, PCW, FSK, DBPSK, CSS or FILE), channel bandwidth, NR subcarrier spacing, modulation, offset from Fc, relative power and random-data seed. + adds a carrier beside the last; × removes one. With a single carrier, Power is disabled — it is relative to the others, and Pavg is the total. Carriers more than three channel bandwidths apart get their own envelope fundamental in the simulation. Click a card to show that carrier on the EVM tab.

A FILE card puts an I/Q file beside generated carriers, at its own offset and relative power. Mixed with LTE or NR carriers, the file must last a whole number of 0.5 ms slots.

Pulsed CW

A PCW card is a pulse train: a CW carrier switched on and off, the stimulus a pulsed-radar amplifier is specified with. The Pulsed CW preset replaces the carriers with one, or choose PCW in a card's standard list. The record is one pulse repetition interval, which repeats, so the Envelope analysis's Stop sets how many pulses are simulated. Its fields take a number in µs, or a number with its unit (50 us, 0.5 ms, 500 ns):

The envelope is exactly zero in the off-time and exactly one on the flat top. A pulse that cannot be generated is refused on the field it names: a width as long as the PRI, an edge longer than half the width, a pulse that ends past the PRI, a zero Delay, or a PRI that no 1.92 MHz × 2ᵏ sample rate divides into whole samples. Delay must be greater than zero because an envelope run starts from harmonic balance at the first sample of the record: in the off-time that is the quiescent point, so the first pulse starts cold.

Power is the pulse top. A pulsed-CW record is normalised to unit power on the flat top, so the source's Pavg is the pulse-top available power — the number a radar specification states — and Pin_avg_dBm and each IProbe's Pavg_dBm in the results stay averages over the whole run, off-time included. Harmonic balance and loadpull play a MOD_Source as a CW tone at Pavg, which for a pulse is the top: the steady state of a pulse that never ends.

Its bandwidth is its edges'. The faster edge sets the carrier's bandwidth (twice the band holding 99 % of a raised-cosine edge's power, over 0.9, as an I/Q file's is measured), and the envelope rate, the preview's span and the sample rate follow from it. The native rate is the smallest 1.92 MHz × 2ᵏ that puts at least ten samples in the faster edge and a whole number in the PRI.

A pulsed carrier is the source's only carrier: a pulse beside a modulated carrier has no clear power or record. With one on the card, Record, Clip to PAPR, Spectral shaping and + are disabled: the record is the PRI, and a pulse is neither clipped nor spectrally shaped. A pulse draws no random data, so its card has no Seed.

In the netlist the card is

Carrier[1]="PCW;PRI=100us;PW=50us;Rise=0.5us;Fall=0.5us;Delay=10us;Off=0;P=0"

and two keys the card does not show set the droop window: MeasureStart and MeasureEnd, fractions of the flat top (0.1 and 0.9 by default).

Not modelled: modulation inside the pulse (linear FM chirps and phase codes, which real radar pulses usually carry — the natural next addition), and pulsing the gate or drain bias with the RF, which is a pulsed bias source rather than a signal.

FSK

An FSK card is binary frequency-shift keying with continuous phase: each bit holds the frequency at Fc + Dev (a 1) or Fc − Dev (a 0) for one bit period, and the phase never jumps. Choose FSK in a card's standard list, or a mode from the Wireless M-Bus or Bluetooth LE preset.

The line under the fields gives the modulation index h = 2 Dev / Rb, what the settings make, the bits and the record. With a Gaussian filter it is GFSK, and with h = 0.5 it is GMSK: GSM's is 270.833 kb/s, ±67.708 kHz, BT 0.3. Without the filter h = 0.5 is MSK.

The Wireless M-Bus menu (EN 13757-4) sets one FSK carrier on PN9 for a metering mode and moves Centre Fc to its channel. If Fc is an expression, the menu leaves it as you typed it.

C1 meters only transmit; C2 meters also listen, on the other-to-meter link. Only that link is Gaussian filtered. T mode is the same split.

The Bluetooth LE menu sets one GFSK carrier at 2.44 GHz, the middle of the 2.4 GHz band, with h = 0.5 and BT 0.5:

LE Coded (125 kb/s at S = 8, 500 kb/s at S = 2) sends the LE 1M air interface: its coding is in the bits, so an RF front end sees the same signal. Classic Bluetooth's basic rate is the same FSK card at 1 Mb/s, ±160 kHz, BT 0.5.

The data is a test pattern, not a telegram. There is no preamble, sync word or 3-out-of-6 coding, because an RF front end responds to the modulation and does not depend on what the bits mean.

The record is one period of the data: 5.11 ms for PN9 at 100 kb/s, 80 µs for an eight-bit pattern at the same rate. It repeats, so the Envelope analysis's Stop sets how much is simulated. Over a record the phase gains π h for each 1 and loses it for each 0, and that rarely adds up to a whole turn: PN9 has one more 1 than 0s. So the record is closed by moving the whole carrier by at most half of one over the record, 98 Hz for PN9 at 100 kb/s. The seam is then as smooth as any bit edge.

Bandwidth and rate. The carrier's channel bandwidth is Carson's, 2 (Dev + Rb): 290 kHz for C1. The preview's span, the envelope rate and the ACPR bands follow from it. The record is sampled at Rb × 2ᵏ, at least 32 samples per bit and eight times the bandwidth. A 511-bit record holds a whole number of samples at none of the standard sample rates, so while an FSK carrier is on the card the Source box's list offers Native rate, second after Auto, and Auto uses it.

FSK has a constant envelope: its PAPR is 0 dB, and its CCDF is a step at 0 dB. As with a pulse, it is the source's only carrier, and Record, Clip to PAPR, Spectral shaping and + are disabled. A PN sequence draws no random data, so the card has no Seed. The second preview tab reads Frequency and shows the instantaneous frequency over the first 64 bits, where the Gaussian filter's rounding of each bit edge can be seen.

In the netlist the card is

Carrier[1]="FSK;Rb=100kbps;Dev=45kHz;BT=0.5;Data=PN9;Off=0;P=0"

Not modelled: transmitting in bursts with a duty cycle (a metering transmitter sends a few milliseconds every several seconds), and AM or PM.

DBPSK (Sigfox)

A DBPSK card is differential binary phase-shift keying at a low bit rate: the Sigfox uplink, an ultra-narrowband (UNB) signal at 100 b/s in the 868 MHz band (RC1). A 0 reverses the phase and a 1 keeps it. Choose DBPSK in a card's standard list, or the Sigfox preset, which also moves Centre Fc to 868.13 MHz.

The reversals are smooth. Each bit is a raised-cosine pulse two bits long, centred on its bit, so two equal bits in a row hold a flat envelope and a reversal passes through zero along a cosine across the bit edge. The envelope therefore dips at each 0 and nowhere else: the PAPR of PN9 is 1.25 dB. Phase flips with no shaping would spread 99 % of the power over ±9 Rb; shaped, 99 % lies within ±0.7 Rb, and the channel bandwidth is the pulse's main lobe, 2 Rb (200 Hz at 100 b/s). The record is real-valued.

The record is one period of the data, or two when one period holds an odd number of 0s: the phase would then end reversed and the record would not repeat. PN9 has 255 0s, so its record is 1,022 bits, 10.22 s at 100 b/s. The record is sampled at 32 samples per bit (3.2 kS/s), which the Source box offers as Native rate and Auto uses. A standard rate is refused for so long a record. The envelope analysis steps at that native rate too, not at the 1.92 MHz the LTE and NR ladder starts from, so 50 ms (five bits) is a short run.

The second preview tab reads Waveform and shows the in-phase waveform over the first 32 bits. The spectrum's offset axis reads in Hz. As with FSK, a DBPSK carrier is the source's only carrier, it is not clipped, and the card has no Seed. The data is a test pattern, not a frame: a Sigfox uplink frame of 12 bytes lasts about 2 s, and a message sends it three times on three frequencies within its 192 kHz band, with gaps between. For a power amplifier, one continuous record is the same signal.

Carrier[1]="DBPSK;Rb=100bps;Data=PN9;Off=0;P=0"

Chirp spread spectrum (LoRa)

A CSS card is chirp spread spectrum, the LoRa modulation that LoRaWAN runs on; its EU868 channels are 125 kHz wide. Each symbol is one chirp: the frequency sweeps the bandwidth BW once, up from Fc − BW/2, in 2^SF / BW. A symbol's value, SF bits, is where on the sweep it starts; it wraps from the top to the bottom when it gets there. The phase never jumps, and the envelope is constant: the PAPR is 0 dB. Choose CSS in a card's standard list, or SF7 or SF12 from the LoRaWAN preset, which also moves Centre Fc to 868.1 MHz.

The line under the fields gives the symbol, the raw bit rate SF × BW / 2^SF (before coding) and the packet. The record is one packet: 8 preamble up-chirps, two sync symbols, 2.25 down-chirps that mark the start of the frame, and 32 payload symbols read SF bits at a time from PN9. That is 44.25 symbols: 45.3 ms at SF7 and 125 kHz, 1.45 s at SF12. There is no header, coding or CRC, as an RF front end does not depend on what the bits mean.

The channel bandwidth is BW. The record is sampled at 8 BW (1 MS/s at 125 kHz), offered as Native rate. The second preview tab reads Frequency and shows the instantaneous frequency over the preamble, the sync symbols, the start of frame and the first payload symbols: a sawtooth that turns down at the start of frame.

Carrier[1]="CSS;SF=7;BW=125kHz;Off=0;P=0"

BW takes Hz, kHz or MHz, and is 125 kHz when absent; SF may be 5 to 12 in the netlist.

The spectrum and the CCDF

The Spectrum is the composite power spectral density against the offset from Fc, framed on the occupied channels; for a pulsed-CW source it is the train's. The CCDF is the probability that the instantaneous power exceeds the average by more than a given margin.

• The Gaussian reference. For a noise-like signal (LTE, NR, several carriers, an I/Q file) the CCDF is drawn against the CCDF of Gaussian noise, dashed and named in the plot's legend. Unclipped OFDM lies almost on it, and a clipped signal's curve bends down well inside it. A pulse, FSK, Bluetooth LE, LoRa or Sigfox signal is no kind of noise and never approaches the reference, so it is drawn alone.
• A constant envelope (FSK, Bluetooth LE, LoRa, a gated pulse) is a step at 0 dB: its PAPR is 0 dB, and an amplifier can run it in saturation. A Sigfox signal's step is at 1.25 dB, from the dip at each phase reversal. A step this narrow is framed from 0 to 12 dB, so it reads as a step.
• A pulse is gated by default. With CCDF over the pulse on-time only (gated) ticked, beneath the plot, the CCDF and the PAPR are measured from the rising edge's 50 % point to the falling edge's, against the pulse's own average, as a signal analyser gates a burst. The plot reads CCDF (pulse on-time) and a flat-topped pulse is a step at 0 dB. Untick it to measure over the whole PRI, where the off-time is in the average: the step then sits at the peak-to-average, which is 1 / the power duty (10 dB at 10 %). The line beneath the plots gives both. The setting belongs to the Designer and is not written to the source.

Below the plots: the carrier count and instantaneous bandwidth, the record and the rate, the PAPR at 0.01 % and the peak, and each carrier's ideal ACPR.

The EVM tab

For each LTE or NR carrier, the EVM tab demodulates the record the source plays (after clipping and shaping) and compares every resource element with its ideal QAM point. One complex gain is fitted over the record, with no per-subcarrier equaliser. The tab shows:

• the rms EVM in % and dB, and the peak EVM;
• which signal was measured: carrier, standard, bandwidth, modulation, offset, seed, record and clipping. The tab has no picker of its own. Click a carrier card on the left to show that carrier, and press Calculate to generate the signal on the left and measure it again;
• the chosen carrier's constellation over its ideal points;
• the envelope power around the record's peak, with the average and the peak as flat lines. A black arrow between them is labelled with the peak-to-average ratio. The Window field sets how long a stretch is shown (2 µs by default). It only redraws the plot, and Apply never writes it to the source.

An unclipped signal reads close to 0 %. Clipping is what moves it. CW and I/Q-file carriers carry no reference symbols, so they have no EVM. A pulsed-CW signal has none either: for one, this tab reads Pulse and holds the pulse itself, |envelope| against time over one PRI, which zooms onto an edge like any plot. For an FSK or CSS signal it reads Frequency and holds the instantaneous frequency, and for a DBPSK signal Waveform, the in-phase waveform.