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HAMEG HMF2550 50MHz Arbitrary Function Generator This signal generator can deliver a 14-bit arbitrary waveform at 250 megasamples per second, a sine or square wave up to 50MHz or a triangle wave up to 10MHz. It can modulate the amplitude, frequency or phase by another generated or external waveform. It also does Pulse Width Modulation (PWM), Frequency Shift Keying (FSK) and more. B ecause the Hameg HMF2550 is an arbitrary function generator, it can produce practically any repetitive waveform shape with up to 256,000 distinct points. Generating sine, square and triangle waves is easy since they are pre-programmed and accessible via dedicated front panel buttons. The square wave has an adjustable duty cycle while the triangle wave has adjustable symmetry. It has several other wave shapes stored in ROM such as sawtooth, noise, cardinal sine (“sampling function” or “sinc”) and exponential sawtooth. User-defined waveforms can be stored in RAM or on a USB flash memory drive. They can be entered manually via the front panel (a tedious process), saved from a computer or captured from a Hameg oscilloscope. It also has a pulse output mode which is ideal for synthesising signals compatible with digital logic inputs. The unit The HMF2550 is housed in a slim, attractive case about two rack units high. The control panel is uncluttered despite the many pushbuttons, some of which illuminate to show the current mode. The display is a 9cm colour TFT Review by Nicholas Vinen 72  Silicon Chip siliconchip.com.au LCD which is small but also bright and sharp. Three BNC connectors are mounted on the front panel – the signal output, the trigger input and the trigger output. The trigger output is useful for synchronising an oscilloscope or another signal generator. There is also a USB connector for connection of flash drives containing custom waveforms. There are four more BNC sockets on the rear panel – the external modulation input, the ramp output (more on this later), the 10MHz frequency reference output and a frequency reference input for synchronisation. There is also a second USB port for connection to a computer along with an RS-232 serial port and the mains power socket. Accessories supplied include the power cord, user manual and software CD. User interface In general the HMF2550 is easy to use. Its major modes are directly accessible via dedicated, illuminated buttons. The TFT display shows the the current generator settings as well as a rough depiction of the output waveform shape. The Sine, Square, Triangle, Pulse and Arbitrary buttons select the main output mode with a single press. Another three buttons enable Modulation, Sweep or Burst (one at a time). Central to the front panel is the sixteen-button keypad used to enter values (frequencies, voltages, times, etc). Value entry is made simple by the four unit scale buttons to the right of the digits. For example, to enter a frequency, you type a number and then press either “MHz”, “kHz”, “Hz” or “mHz”. This is intuitive as numbers can be entered in whatever scale you prefer. These scale buttons are labelled with other units too. So if a voltage is being entered, they become “V”, “mV”, “dBm” or “%”. For time entry they become “ns”, “s”, “ms” or “s”. Alternatively, these values can all be varied by rotating the knob, although that is really only useful for small adjustments. Generally the knob (and the four arrows arranged around it) is used to select the field to be manipulated. The three quick access buttons above the output BNC socket are a nice touch, allowing the user to toggle the output on or off, enable or disable the output offset voltage or invert the output signal with a single press. The remainder of this unit’s functions are accessed via the menu button and five “soft” buttons arranged alongside the display, whose function changes depending upon the current mode. Much of the time they are used as short cuts to select a field to be manipulated (frequency, amplitude, modulation type etc). Unfortunately, some of the functions do not respond instantly to button presses. There can be delays of half a second or more when switching modes but simple functions such as changing the frequency or amplitude via the keypad are quite fast so it is generally not a major issue. Fig.1: the amplitude modulation feature in action. The 20V peak-peak 1MHz sine wave is being modulated by a lower frequency triangle wave over 100% of the amplitude range. Fig.2: here the generator has the same settings as in Fig.1 but using frequency modulation, with a large amount of frequency deviation to make it more obvious. Features The first extended mode is Modulation, where a second waveform can be used to modulate the signal. This waveform can be another arbitrary waveform or supplied via the analog modulation input. The supported modes are amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency shift keying (FSK) and pulse width modulation (PWM). In each case, the amount of modulation can be adjusted. siliconchip.com.au Fig.3: this image shows how the ramp output (blue trace) and trigger output (green trace) operate in sweep mode. The sweep is a 20Hz-20kHz sine wave at full amplitude. August 2010  73 The rear panel has the USB and RS-232 interfaces along with modulation input, sweep output and 10MHz reference input and output. In AM mode it is specified as a percentage of the amplitude, in FM mode the maximum frequency deviation, in PM mode the maximum phase deviation, in FSK the frequency hop size and in PWM mode the duty cycle percentage variation. The second extended mode is Sweep, where the signal frequency smoothly changes between the start and stop frequency with either linear or logarithmic timing. Simultaneously the ramp output sweeps linearly from 0V to 5V (see fig.3). This can be captured by another instrument and used to plot the frequency response of the device under test. The third extended mode is Burst, which repeats the waveform a specified number of times in a given time interval (see fig.4). Alternatively, the signal can be “gated” by an external source; ie, whenever the gating signal is low output is disabled and when it is high the output is enabled. The Pulse waveform functions differ from the other modes. When Pulse is selected, the output level varies between 0V and the specified voltage (say, 5V). The rise and fall times can be specified, as can the duty cycle. In this mode, only Pulse Width Modulation is possible (see fig.8). PWM can not be used in any other mode. As with other modulation modes, an internal or external signal can be used. The result is a pulse train at the specified frequency and average duty cycle, with the duty cycle varying with the modulating waveform level. This could be useful for testing switch-mode power supplies, motor control circuitry, Class D amplifiers or other such devices. Fig.4: burst mode, configured for 10 repetitions of a 1MHz sine wave every 33µs. The blue trace is the trigger waveform. Fig.5: the square wave output at 1MHz. There is some ringing after each transition but the rise and fall times are insignificant at this frequency and there is little rounding. 74  Silicon Chip Software The provided software allows for simple waveforms to be created or edited. It loads and saves CSV (Comma Separated Value) files which contains the co-ordinate data for the waveform. The files can be loaded onto the HMF2550 via the USB or serial interface, or by saving them onto a USB flash memory drive. However, the most likely source of arbitrary waveforms will be those recorded on an oscilloscope or mathematically generated. Since the HMF2550 accepts data in the common (and easy to create) CSV format, it is possible to convert data from many Digital Storage Oscilloscopes into a format siliconchip.com.au that the HMF2550 can handle using a spreadsheet program. The firmware can be upgraded via the USB flash drive interface. It is a good idea to keep the firmware up to date in order to take advantage of all the bug fixes and feature upgrades. Performance The sine wave output is visually undistorted from below 1Hz up to 50MHz. The specified harmonic distortion level is <0.04% up to 100kHz. We made our own sine wave distortion measurements at maximum amplitude (20V peak-to-peak) with a 10Hz500kHz measurement bandwidth and they are shown in the table below. In summary, our measurements are less than half the maximum specified distortion level. The signal to noise ratio under the same conditions is 92dB (unweighted). Frequency  THD+N Ratio 100Hz 1kHz 10kHz 100kHz 0.0127% 0.0128% 0.0140% 0.0180% Signal Amplitude (peak-to-peak) 19.800V 19.924V 19.896V 19.670V With square wave output, the rise and fall times are below 8ns so it remains fairly rectangular up to 1MHz. Between 1MHz and 10MHz it becomes more trapezoidal, with increasing ringing after the transitions and above 10MHz the signal becomes progressively more sinusoidal. The maximum triangle wave frequency is 10MHz but distortion is visible at 3MHz and becomes progressively more significant. The output voltage swing and drive strength are good with up to 20V peak-to-peak into light loads and 10V peakto-peak into 50Ω loads. It is possible to improve the frequency accuracy by feeding in an external 10MHz frequency source, but generally this is unnecessary due to the excellent temperature stability (±1ppm from 18°C-28°C) and excellent aging characteristics (±1ppm over one year) of the unit’s own reference. In fact the HMF2550 can be used as an accurate 10MHz reference clock source for other instruments. Fig.6: a 5MHz triangle wave, which is half of the maximum supported frequency. Some distortion is visible near the peaks – much more than there is at 1MHz. Fig.7: this is the maximum sine wave frequency provided by the HMF2550. Conclusion This is a very flexible signal generator which is easy to set up and use. The wide range of frequencies and amplitudes, and the ease with which user-defined signals can be integrated means that this instrument will meet a wide range of needs. Overall the interface is well designed and intuitive, although if you press the wrong button it can sometimes be confusing to get back to where you were. The most impressive aspect is the flexibility provided by the modulation options. The pulse and sweep modes are not quite as comprehensive as they could be but in reality this device can perform all the common analog signal generation functions that are needed within its supported frequency range. The HMF2550 (and 25MHz HMF2525) are available from Rohde & Schwarz Australia. Prices range between approximately $1900 and $2450 depending upon the model and configuration. The standard warranty is one year. Call (02) 8874 5100 or e-mail Sales.Australia<at>RohdeSchwarz.com for more information. SC siliconchip.com.au Fig.8: the pulse mode output at 1MHz. Note that it is not centred about 0V. It has been set for 10V peak amplitude and is being pulse-width-modulated by a sine wave. August 2010  75