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Terminal lets you drive RF signal generator

Posted: 21 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:signal forge? test equipment? 800 signal generator?

Signal Forge LLC was founded last year specifically to develop test equipment that comes in small and affordable packages. Significantly, the company's Signal Forge 800 signal generator, which is its first product, was the result of solving problems that the firm's founders faced repeatedly in circuit board and IC design labs over the past 20 years. They wanted to accelerate product development on a limited budget.

As a synthesized generator based on a TCXO (temperature-compensated crystal oscillator), the low-harmonic and low phase-noise SF800 combines several key functions. It can generate specific frequencies, as well as sweeps. It also handles FM (frequency modulation), AM (amplitude modulation), and arbitrary modulation. Operating power is derived from a small wall wart-type brick.

Decent specs

It will generate signals with a resolution of 1-Hz, and with an amplitude resolution within 1-dB. It's stable, too. Long term stability is specified to drift no more than 3-ppm in the first year, and then no more than 1-ppm/year thereafter. Short term stability is just 0.0002% after a brief warm-up.

Two models are also available, dubbed the SF800 and the SF800E. The only difference between them is that the 800E version can be driven by an external reference standard, and it doesn't have a clock output. The price for either is the same, however.

Terminal software

Not mentioned in Signal Forge's press release (on the left) is the fact that this system works in conjunction with a PCor even a simple terminal.

All you need do is connect the box to your computer's RS-232 port using a standard serial cable with a DB-9 connector. Alternatively, the SF800 can be connected using USB (Universal Serial Bus). However, you'll need a serial port-to-USB adapter. Signal Forge recommends an in-line Keyspan 19HS adapter for the purpose. The Keyspan unit, typically selling for about $30, supports data rates up to 230kbps, which is twice the speed of most PCs serial ports. It's also a plug-and-play peripheral that draws its operating power from the USB connection.

Once connected, you then run the company's Wave Manager software. It handles the SF800 generator's set-up and configuration as well as menu-driven waveform programming. The software runs with standard terminal console programs such as Windows HyperTerminal.

At power up, a main menu is displayed, presenting the current configuration, operational status and programming options. The menu screens are organized such that only the options allowable for the selected waveform and output type are displayed. That makes the software easier to learn and navigate.

The Wave Manager software also gives you an option to save waveforms and then load them at power-up. This is a nifty feature that lets an SF800 operate in a repeatable stand-alone mode, which could prove useful in the hands of semi-skilled or production operators.

Custom signals

In any case, the Wave Manager lets you generate a range of signals and modulation features. Waveform modifiers are applied to customize the output. In addition, you can develop arbitrary modulation files to create your unique FSK (frequency shift keyed) signals, or AM outputs. An arbitrary modulation option is also displayed on an Edit Parameters screen of any waveform that supports user-developed waveform modulation files.

While either single tone or FSK waveforms are running, you can change the frequency by stepping up or down in discrete steps using selections on a Modify Active Waveform menu. The frequency can be repeatedly stepped up or down in increments of 1-, 10- or 100Hz. You can also enter a new frequency value.

The Wave Manager software also lets you change the base power level and offset during operation (when AC coupling is enabled). Power values are entered in units of dBm. Like frequency, power can be increased or decreased, over a -11dBm to 7dBm range, within an accuracy of 2dB. For its part, the SF800's offset feature lets you adjust the base power to offset external losses such as cable losses.

Multiple outputs

Back to the hardware. A dedicated output port also supports testing of digital systems. The unit supports 3.3V TTL, as well as 2.5V TTL, and 1.8V STTL.

For its differential outputs, two SMA connectors are provided for driving the differential clock output. The system's LVPECL-compatible 50MHz to 800MHz range is usable to a GHz. The unit's AC-coupled output uses one SMA connector. The sinewave signal at this port has a frequency range spanning 100kHz to 800MHz, although it's also usable to 1GHz. On the digital side, a BNC coaxial connector delivers the generator's TTL, LVTTL, and STTL signals over a frequency range from 1kHz to 100MHz.

The SF800 generator also supports a measure of external control. A dual-row 10-position header provides ten two-pin connectors that let you switch the system's differential clock on or off (for OOK (on/off keying) applications, as well as FSK or ASK (amplitude shift keying). When you're using the generator's external control pins for frequency control, each pin has a lowpass filter at its input (except the Start pin). The filters have 3-dB cutoff points of 4.8-kHz.

Remote start operation

The header also has hooks to support remote start operation. A high to low transition is all that's needed to enable the SF800 to start outputting a pre-programmed frequency on the system's AC-coupled, differential, and TTL outputs.

There is a time-from-assertion to output-valid delay, and this can vary depending on the type of output/frequency chosen and modifiers used, so you'll have to measure the delay and account for it if the latency might affect the type of test you're performing. It's not much of an obstacle, though, especially considering the SF-800's favorable cost-vs-performance ratio.

When you consider its less-than-$1000 price tag, and its excellent stability and low phase-noise outputs, an SF800 shapes up as a cost effective alternative to many higher priced unit. In fact, it's so reasonably priced that you may want to consider using more than one for some applications.

- Alex Mendelsohn


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